I agree with what has been said.
Just turn off guest posting and also enable image verification for registration. For the image verification, GD must be installed on the server, but it probably is already.
I'm guessing Jamil knows how to do all of the required changes, but if he needs help, I'll reinstall phpBB on my local machine and post screenshots or somethnig.
By the way, if there is some bug in the current forum that won't allow for the necessary changes to occur, I'd be willing to donate a vBulletin license I have to TBN.
VBulletin is a bit trickier to administer, but at the same time it is a lot more powerful. Also, it'd allow you to propagate all of your old posts from phpBB.
- joe