Presumably, like stealing someone's car keys if they're trying to drive drunk, or taking someone's stick on the city streets if you're one of Louis Rossman's crew in NYC and the perp is going around smashing stuff ;)

I think your presumption is illogical, since you take someone's car keys to stop them from committing a crime and harming/killing others.

TL;DR It probably wouldn't happen literally, but any local community is too small and interdependent.

I've never actually seen this threat "executed": the rules are all unwritten, but most people get a clue when reminded or lectured. My friend once wanted to skate in a broken helmet - it literally saved his life a few days before. We had a cute argument, he said it's not really broken if he only hit his head very lightly (so "lightly", he actually had a slight neck pain). I told him I'd buy that broken helmet from him at a full price, just so he doesn't use it, he wouldn't believe me until I took €150 out of my pocket. Eventually he agreed to not be an idiot ;)

If you'd insist on not wearing a helmet? On an official event, you'd probably get removed, possibly banned from future events. You need a sticker with your number on the helmet, so you wouldn't make it as far as the start line. I guess on an unofficial freeride, you just wouldn't get invited again - it's all invite only to begin with, and doesn't make much sense unless you have at least 3-4 people participating; someone needs to shuttle, you split the gas cost, sometimes you need spotters, etc. Randoms on a local spot? You either already know them (so you are allowed to yell at them), or they're new, so you make friends and they get a mentor.