We had PHPBB on the MISoft website for a while, but it was
horrible. Chris Franklin and I were clearing out dozens of pornbot and prescriptobot threads every single day. Since then we've switched over to SMF, and we haven't had a single bot problem using it.
PHPBB is suspectible to bots because your forum actually says "PHPBB" in its URL. Bots hunt down anything called "PHPBB" and storm it like it's Normandy. It has horrible "security features," which did more to keep human users away than to actually stop the bots.
The switch seriously killed us, too. We had about 15 human users on PHPBB, and when we switched to SMF, we now only have 3 users (counting myself and Chris!). I think some of them just gave up on the forums after so many bot attacks. Seriously, don't use PHPBB, I have had nothing but horrible experiences with it. SMF was easy to install, easy to mod, and the community is actually helpful. I equate my experiences with the PHPBB community with that of visiting gamedev.net. That should speak for itself!
"In an interstellar burst, I'm back to save the universe"