funny thing is, YOU can almost completely stop spam by hardly effecting the users, ... for example
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get moderators (im pretty disappointed that no one has responded to my pm about me volunteering)
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replace that dumb text footer with an image, change tags to remove all traces of yabb in plain text (including the darn header) these bots scan google for this exact forum system cause they know how to attack it, this site screams NOOBIE with a vulnerable system
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check user emails upon registration against a spam blacklist, like http://www.stopforumspam.com/
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clean up the mess, bots are looking for post's by other bots
All of these are transparent to the user, and you can stop the spamming almost instantly
All it takes is a bit of time and some effort