I have quite a bit of experience with phpbb as you are using on this site. Bot spammers are quite a problem. there is a constant "work in progress" over at phpbb.com with solutions for this. I have had pretty good luck implementing a combination of e-mail activation as well as a picture authentication during registration (where the user has to type in the code as seen in the picture) if I can be of any assistance, please don't hesitate to let me know. I have been lurking on here a while, reading everything just not posting a lot
