just cleaned up the forum but this is becoming a real pain! The amount of spam just keeps increasing despite the use of a CAPTCHA image...
You are running an out of date version of phpBB which has a number of exploits in it that allow spambots to post. Upgrade to the latest version, and in addition insist on administrator approval for new members to avoid 'human' (I use the term loosely!) spammers. You can usually easily spot the spammers from the real users, and just don't approve them so they can't post. There are some handy mods for phpBB which make the task of going through the user list and deleting the spammers a bit easier. See the user forums on www.phpp.com