Its a real nightmare with the forum spam. I check the site every morning (British time) so like you say it gets removed pretty quick. When I find spam I ban the IP address it came from and remove the offending user. The user registration form is bot-proof so I think what happens is that human spammers register new user accounts then feed these into a spambot which posts the offending items. Ill install a new forum module over next week or so in order that all posts have to be human-verified by typing in the characters from an image, as this method seems bot-proof.
Half the problem I think is the software I use, from what I gather PHPBB is so popular now that any popular phpbb site is a spam magnet. Ill try installing some more anti-spam modules before thinking about changing forum software though!