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.
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.
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?
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.