Here I'll plug
SMF; it's feature set is more comparable to vBulletin than something like phpBB ( no subforums? no attachments? can't merge topics? come on =/ ), and if you go on Converters in the download section, there's a phpBB > SMF Converter.
I forget if phpBB even has captchas (where it generates a random image and you have to type the letters you see to verify you're human), but if it does, you should enable that.
The last phpBB forum I had anything to do with was plagued by constant signups by automated bots posting ads for pharmecutical products and online texas holdem poker and the like =/
I run a forum that used to be phpBB and we eventually converted to SMF, and we've never looked back
Jam it back in, in the dark.
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Roph