bans work either by banning a single, specific IP, or by user account. don't know if the forum has the capability, but another common more extreme measure is to ban an IP range, but then that blocks other addresses that are innocent.
iirc from some conversations with Jack some time back, he got himself some IP-spoofing software. there are all kinds of programs out there like that, free ones that probably use you as part of a botnet (lol!) and cheap ones too. so it's easy to get around an IP ban anyway. online multiplayer gaming, i think in particular, has helped create a big market for things like this.
something that some game servers can do that's stronger than an IP ban is to ban by hardware device ID -- so that a gamer that's been banned from a server has to change video and and/or hard drive before they can reconnect. i don't know if any webforums can do that sort of thing at all, since they don't interact with users via installed software like games do, they wouldn't do it in quite the same way. (?)