Shame

Ever wondered why Santa Claus goes "ho ho ho"? Turns out it means exactly what you think it means. Santa keeps an additional naughty list of unfaithful married couples, especially those high in the political ladder and the military chain of command. One wrong move and the whole world gets to find out how much of a ho-ho-ho you or your spouse has been

Alternative "countermeasures"

For would-be grinches lower in prominence, Santa has other methods. Let it not be said that he's a one trick reindeer.

Santa has been utilising psyops long before there was even a NORAD. The commercialisation of Christmas was part of his master plan. All the trappings of Christmas, like gifting, endless jingles, Whamageddon etc, are a sustained DDoS(Distributed Disruption of Surliness) attack waged by the corporations who are really Santa's proxy botnet.

On the big day itself, Santa deploys his air force to clear the way. Using their powerful ECM(Extremely Childlike Merriness) packages, highly trained elf pilots flying dedicated strike festives perform SEAD(Shutting-up Extremely Annoying unDesirables) missions. They are supported by AWACS(Archaic Wassailing And Carolling Sleighs) craft who perform command and control duties over the airspace. These provide guidance and help out with their ECCM(Eggnog-Caused Conclusive Meliorism). NORAD never stood a chance.

Santa is the last active user of the SR71 defence mechanism, he's just too fast.

A quick and dirty calculation of how many deliveries he has to make in a single night, and the distance he has to travel to make those deliveries, suggests that he's moving far faster than anyone is able to intercept.

We might be able to track him, NORAD may know exactly where he is, but they've got nothing actually able to hit him.

Santa is protected by mutual ensured destruction, the same force which arguably protected us during our nuclear infancy.

It works like this...

Santa is the only reason why children even attempt to stay off the naughty list.

Without that incentive, those little rug rats would be completely uncontrollable.

Destruction doesn't even begin to describe what they would do if anyone took out Saint Nick.

Think about this, as parents, you are the source of every disciplinary scolding and every bottom swat that your children have ever received. ...and they know where you keep the butcher knives.

NORAD doesn't admit it, but they send up a wing of fighter jets to escort and protect the sleigh while it is over US Protected airspace. Most of the other countries do the same, and CIA drones cover those countries which lack the good sense to put up their own defense.

Santa does not have to worry about protection because the rest of us grown-ups worry for him. That is why parents always look so weary on Christmas morning. They have been up all night wishing...

Merry Christmas To All and To Nick A Safe Flight!

Counterintelligence and stealth technology.

It is well established that Santa has to travel at the speed of fantasy to be able to deliver all the gifts in time around the world. The only way to catch him is using air force and radar.

First of all his sledge is built with the latest stealth technology, which most of the radar cannot track. But the guys of NORAD have the technology to follow their own stealth planes, so something more is needed.

His chubby appearance is a clever disguise for a secret anti radar technology, well hidden under the red coat, which offsets the radar echo tricking the NORAD into giving a false position.

And just as additional measure, he makes sure to deliver the gifts first to the party-poopers, so that while they are busy unwrapping he can visit the rest of the area.

Nobody can actually track him

It's been established that the speed and route that Santa has to take are, to put it lightly, absurd. But that's not any fun, and Santa knows this. To help him keep the christmas spirit in children while also accomplishing his job, Santa employs a significant number of look-alikes, drones, and other equipment.

Some just need to look like a sleigh and reindeer, to let the children catch a glimpse. There's no way they'd be seeing him at an average speed of ridiculous, so these let them enjoy the magic while he does the actual magic.

Some are more complex. Maybe with an additional set of reindeer and an elf in a replica sleigh. Or, more accurately, a replica of what people think the sleigh looks like. The actual sleigh is probably a bit different, given the abuse it has to endure.

And there's even some that aren't meant to be seen by eyes. They give off the correct radar signature, and one of these is famously tracked by NORAD.

Meanwhile, Santa zips around in an undetectable fashion, possibly breaking the laws of physics and/or time. While he knows he is perfectly safe on those counts, he likes the way people think of him, so he takes efforts to make sure he doens't spoil the magic. Plus, if a kid is awake, a well-timed drone outside the window can give him the opportunity to sneak in, eat the cookies, drink the milk, and drop off the presents without being seen.

What NORAD detects is a drone bogey to keep them occupied, while he uses a more discreet method of travel.

And for that matter, the sleigh doesn't have infinite capacity, the sack does.

Santa plays by Mutually Assured Destruction rules.

We see the one sleigh he normally uses, but anybody who runs operations like this one has backups capable of completing the entire job just in case something goes down.

Santa has to make not less than 233,000,000 stops, and travels between houses at an average speed of 1,800 miles per second. If we assume a smooth acceleration curve his peak velocity is double, but to make it more fair for the militaries of the world, we won't do that.

Santa's sleigh must therefore possess 838,800,000,000 miles per second of delta V and some stupid huge TWR. If this is provided by fuel at all, it had better be some kind of mass cancelling magical storage because if it's ordinary antimatter I'd be amazed if the calculations don't work out to this being a planet buster. If someone takes him out, it had better be a clean kill, or any remaining reindeer are going to be panicked and ludicrously dangerous to approach.

In any case, the elves with the backup sleighs will have no significant trouble demolishing the military might of the world by shockwaves alone. Somehow, Santa is cancelling his shockwaves from the stupid huge acceleration to keep the cities from being blown away. All the elves have to do is don't.

Since Santa and the reindeer live at the North Pole for so long, they have evolved ways to not radiate so much body heat, internalizing it as much as possible. This makes it extremely difficult for heat seeking armaments to lock onto them.

For radar... well, that sack probably has a ton of tinsel that can be used as chaff.

And then there's the whole prospect of going down in history as the person that ended gifts for the whole world. Can't say that would make one popular. I highly doubt anyone will buy you a drink at the bar if you're the one that offed Santa.

People have tried to sneak up on Santa. As they have found out, he sees them when they're sleeping, he knows when they're awake.

Santa already has defenses! While their purpose is simply protection from the environment in which he operates they're plenty strong enough to shrug off anything the military of any nation can throw, up to and including a direct hit with a nuclear weapon. Even the biggest booms are nothing compared to the atmosphere at the speeds he must be moving in order to make his deliveries in a day and a half.

With his gifts

Santa is a cunning man. As a maker of toys he has a natural supply of decoys, including other fake pop-up Santas and reindeer. And although NORAD thinks they have him tracked, it's actually a decoy transponder that he plants on a lookalike sleigh.

Equipped with an arsenal of loud noisy things, insta-pop-up balloon Santas, disco smokescreen machines, loud fireworks, he's the master of misdirection and distraction. After all, how do you think he gets past all those eagle-eyed children who are looking out for him?

Santa has another gift: he can see into people's hearts, and knows what they truly want, that's how he knows they've been naughty or nice. Even the most hardened military soldier is going to melt when they find Santa brought them something precious, like a card from a loved one, photograph or even a puppy. Sargent might not be too happy.

With some kitted out defence tech

But automated systems can't be bought with gifts. What's Santa to do when his sleigh is undoubtedly detected on radar and IR-tracking missiles? Santa has some close friends in the military who've kitted out his sleigh with chaff, both heat-producing and radar baffling (electronic chaff), of which Santa has modified to also launch fireworks and sparklers and other heat producing sources. He's a jolly fellow like that.

He's also managed to get some electronic system scramblers, so when he's closer the radar systems will be jammed.

With favours

Santa is a popular guy. He's helped many out of a bad situation, and some even owe him a few favours. Blackmail is beneath Santa, as a man of integrity, but he knows a few people who owe him some favours (he once got a stretch armstrong for a guy when it was sold out, long story).

Some of them might be able to provide distractions. There's one kid who is really good with tech, can hack computer systems remotely (he need not travel with Santa, internet is a wonderful thing and Santa has built in wifi because what sleigh doesn't?).

Maybe a few people might read between the lines and knock out a few guards. Santa doesn't approve of course, but as it's not Santa it'll pass.

With reindeer

Now, Santa isn't allowed to go around beating up the occasional bad guy, but Blizter, he's got a real bad attitude, he's really grouchy. Well, all the reindeer can be if you get between them and Christmas.

With antlers and their own mind (almost like they know what they're doing) they might opt to... intervene. They are magic reindeer after all.

With a child ambush

Of course, children love Santa and if they see a jerk trying to harm Santa, well, they're about to get jumped by an ambush of children. Shin kicking, direct hits to the groin, eye poking, make up powder in the eyes, enough to make any wrongdoer think twice.

The real meanies

Santa's not really one to hurt anybody. But then there are the real meanies, the people whose hearts can't be melted with puppies, can't be fooled with plastic decoys or dazzle, the big bad, the grouchy potato, the Grinch, if you will.

And these Grinches are mean, they don't talk, they don't mince words. If it's a problem, its gone, pop, blam, kapow. Reindeer, no problem. Some hacker in pajamas, weak! Children? They eat children for breakfast! How does Santa deal with a savvy bad guy?

With medicine

Well, Santa doesn't deliver just toys. He delivers gifts for all sorts of people, including much needed medicine to old folks and sick children. Of course, Santa has this one troublemaker elf who regularly stows away with the gifts, because elves aren't normally allowed to come along, precisely because of pranks like this.

Because there's infinite storage in the sleigh, there's no way Santa could ever find him. So this joker elf loves playing pranks on people, especially Santa, so of course he takes some laxatives which were going to be delivered to the old people's home, and adds it to the mince pies Santa collects from children, which Santa plans to eat later.

Now these aren't your usual laxatives. They're magic. Fast acting. But because the Grinches are thieves, they steal the gifts and the mince pies (it's why they want to get Santa, he has infinite storage and infinite gifts! It's a magic sack that negates weight, quantum physics, that sort of stuff), and of course, to show how evil they are (they don't mess around)... they eat the pies in front of Santa, despite him telling them not to. And there's only one toilet. Cue a fight over who gets to go in first.

So Santa slips away with the gifts to deliver to the rest of the children... he's survived for another year of hijinx at Christmas.

Sheer weight of numbers

Sure you can knock out a few Santas, or even a few dozen but it's like pissing on a forest fire. There are millions of them coming over to be able to complete the round in time. Perhaps a few children will miss out, but with the best will in the world, you're not going to be able to put a dent in their numbers.

Hundreds, maybe thousands of Santas may fall to the Grinches in a hard year, but still they come, millions strong. The swarms of Santas flying over turns the sky dark as they pass, a sight to bring joy to the heart of any child¹.

¹and great fertility to the fields as the reindeer do what reindeer do.

Posted as a separate answer because it's entirely unrelated to my other one