Suebear

There's no real way to permanently ban someone. The only thing that can be blocked is a username/email address. So as long as he wants to keep creating new email accounts (which is easy) and new usernames (also easy), he can keep coming back. I think that IP addresses can be banned, but I'm pretty sure that most home internet companies use dynamic (ie, changing) IP addresses, it's pretty much impossible to completely and totally ban someone.

When it comes to situations like this, the best thing to do is just ignore him, let the moderators take care of posts which break the forum rules, and he'll hopefully get bored and give up. We had a problem with someone like that when I was a moderator, and he eventually quit coming back.