Human Verification

[matt_turpin]

I've recently become overrun with spam-members. I'm thinking they're all bots, but I've tried all three human verification techniques, and they seem unstoppable. I ban one gibberish username a day lately, and if I don't get to them fast enough they spam my boards with sex messages or, because my website is gaming based, WoW gold ads (a human would get the game right, I'd imagine. Not wow). I don't know what to do. I don't mind banning daily, but I'd prefer a preventative measure.

I've used all three human verification techniques.
I've enabled email verification.
I used to have enabled manual new user verification, but it was too hard to distinguish certain bot names from certain legit users, so I gave that up.

You all have bigger, better boards than I do. How do you deal with this problem? It seems like it started overnight and now I can't keep up.