I moderate a forum stateside and the days of blocking IPs are over.
We have a few other ways of identifying previously banned users though (none an exact science).
Would one method be a cookie?
Well, bearing in mind Admin here would most likely prefer that their detection methods weren't discussed, I won't divulge too much (not that it's a massively guarded secret or anything) but users give many things away about themselves when (and more precisely how and with what) they access forums.
Be like asking an audience to stand-up and then asking everyone to sit down that wasn't in born June, doesn't have red hair, writes with their left hand and only has three of their own teeth left.
If someone is still standing, chances are that's the same person who had all those characteristics before.
The fact is that the kind of people who get banned and re-reg are astonishingly stupid, so they tend to come back and do exactly what they did before and expect to not get caught.
Humans are really good pattern-matching machines (actually we're a bit too good for our own good), so a re-reg can usually be spotted a mile away because he posts in the same way and in all the same forums as he did previously.
The admins have a few other "background" tools that provide them with additional confidence that they've hit on a match but 99% of it starts with, "This guy reminds me a lot of someone..."
especially , when they , write like this ,
Yeah, I mod a US forum site and the detection methods are easy.
Strange spelling mistakes, similar typing structures, the way they phrase things, which fora in particular the new users frequent.
It's pretty basic if you have the time to look into what the new users are doing.