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I have a forum section that allows guest posting.

As you have guessed it is being hit by spam. I Would use captcha but i would prefer to have a security question required when making new thread/posts as a guest.

The spam bots are getting around the captcha but not a security question

This might not stop it all, but do you use a plugin called Akismet? If you don't you should really give it a shot. It's pretty awesome against spam.

I installed a plugin called spamalyser http://mybbhacks.zingaburga.com/showthread.php?tid=1006

With some minor changes (lowering the weight of the detected spam), ive had zero spam be able to pass through, on a guest posting allowed section..

this is one of the most effective plugins against spam posting

Yes spamalyser is great but maybe with the help of templete conditional plugin you can add security question