Apart from constant human monitoring their is no fail safe way to prevent all spam. Captcha might slow them down, but they slow down humans even more.
A monitored signup procedure can work, that is when new members have to write a sensible message explaining why they should join and then their membership is confirmed by a moderator. This gets rid of most computer scripts and should greatly reduce the number of “Human Spamers”, espically the ones with a poor understanding of English. Fairly certain that the phpBB supports this feature. But there are other mods which would make it significantly harder for the bots to sign up without forcing the admin to approve all new members, I suspect something like the Registration Auth Code would be a good start. Plus I hate having to wait to have my membership confirmed…..
Certain yellowfin and the other mods have a good idea already on whats available but this could be useful.
I am not overly bothered with the spam and never report it anymore, the moderators seem to do a good job and it is normally gone within a few hours and it is easy enough to identify a new spammer post from a real one.