If it is mostly automated spam, it might help to try and confuse the scripts by inserting traps for them that users can't see (fields hidden via CSS, etc...) http://jkroon.blogs.uls.co.za/it/spam/ahead-of-the-spammers-for-once
Human spammers are a totally different issue... And most solutions that I can think of would either increase moderator load (such as moderating a user's first post, which is probably not supported by phpbb anyway...), or require a lot of manual maintenance (Keyword based filters)... What the spam do seem to have in common: External images (and usually lots of external images) and external links... Quite a few sites ask the users to solve simple maths problems as well, but that could not be too hard to bypass... Gert
