In reply to this post, I have closed down public write-access to all forums. I hope I didn't forget any forum, and that all the new folks care to sign up to get some help.
1- you can add a picture to validate that this user that signing up is not a robot, there are some "hack"s for phpBB to add this feature to sign up page.
2- most of spam users! have a unique password, you can check them in the DB directly and remove them with a simple query.