Wow, the spam bots now attacked the spam bot thread. Awesome.
If need be, I'm sure some of us could watch for spam posts and delete them for you. Otherwise perhaps you can restrict them to only posting in certain boards until they have a high enough post count or something along those lines?
I believe vBulletin has a plug-in to change the CAPTCHA verification to a Question & Answer verification. The bots can scan the CAPTCHA images and decipher them, but they usually aren't smart enough to answer a simple question.