Blue is right - I was going to mention pir8bay too since they're one of the most famous cases where big players like Microsoft, MPAA, etc. went after them and they thumbed their noses by posting the cease & desists public to make fun.

You can't just yank a domain since ICANN claims they're neutral, and back in the day when Network Solutions was the only registrar it might've been easier, but now with registrars scattered all over the world including shady places, ICANN just says "we don't get involved - we leave it up to local laws in individual countries." Youporn's registrar is some wacky one based in germany that doesn't roll over easily from what I read, and if so they'd just xfer to another registrar beforehand, ie one in sweden.

A DDoS attack also isn't as easy or effective as it used to be for various reasons, not to mention youporn has a mesh of hundreds of servers scattered everywhere when I checked, so to be able to hold that many down you'd have to work at an ISP or telecom company, and not in the U.S. either - eastern europe perhaps. Those little russian fuckers in their big coats would want a nice payday to do their nerdy black ops too.

I'd have some ideas to combat youporn if they were causing money out of my pocket, though they'd be fairly unorthodox.