Man, I apologize for all the spam that managed to get posted to these forums! Periodically the hacker community figures out how to automate captcha recognition in some version of phpbb and when they do your forums get filled with spam until you upgrade the phpbb version to something people haven't broken (yet). Sigh.
Lest spam removal become a full time job, I decided to simply turn on admin approval of new accounts. That will slightly slow down the registration process for new users, but it should completely block the posting of spam.