Just a suggestion to help you keep spammers out... Don't use phpBB's account setup system, make your own that puts what it needs into the phpBB user table.
I had the same problem... so I took the signup link of the website, that didn't work. I had to completly remove the signup php scripts that were being used by phpBB to create accounts. I then proceeded to make my own account creation system that just inserts the right stuff into phpBB and it works great now. I haven't had a spammer since.
Maybe this doesn't matter if you are going to vBulletin soon anyway.
