There is a membership script called Amember that works with phpbb – if you install it new users will have to use the sign-up form on Amember that is totally different than the phpbb sign-up form (also the sign-in form is totally different).
It also prevents password sharing so a spammer in one country cant give his sign-in info to a spammer in another country, you can also prevent the same account from being signed-in mutable times at the same time
So far this has stopped ALL of the spam on the forums I run and administer (excluding spammers with pulses)
http://www.amember.com/
~James