I know that they can change the requirements to register and/or allow the webmaster to approve all new members before enabling accounts. If their name is PenisRBig then you may not want to approve em. Also, their e-mail addresses are dead giveaways sometimes. You could also require that people actully post once in a welcome thread or something. That would at least get rid of the autoregistrations.

When you do ban, hit their ip addresses, e-mail addresses or in some cases a range of ip addresses. I do that and I rarely have to deal with serial spammers. I'll also dis-allow certain usernames. Just some suggestions.

If it's a matter of cost and/or time I will do it for free and on a consistent basis. If something happens where I can't, I'll notify you guys so you can get someone else.