I had the same problem as you, it makes a site look unprofessional and any of the built in spam controls don't encourage user posting when implemented .
Anyway there is a module to address the problem: phpBB • View topic - [26/01/07] Anti-Spam: Only Active Members Can Post URLs
basically it stops new users posting a link until they have posted x number of posts or been a member for x number of days. It's not too difficult to install (the most difficult part is you need to add new database fields) and it stopped spam in it's tracks on my forum.

Regarding phpBB 3 - I'd give it a bit of time before I upgrade to it, I have been testing it on a local machine, it looks nice however most of the existing mods won't work on it and it doesn't address phpbb's insistence on using session ids so I see very little point upgrading as you cant upgrade a 2.x phpbb database to 3 yet.