Looks like phpbb has noticed this. Important to turn on the visual confirmation for registering new members to prevent this DoS attack. http://www.phpbb.com/phpBB/viewtopic.php?f=14&t=301711
Two things you should have already switched on/off: Visual Confirmation = On HTML = Off It is also a good idea to .htaccess your admin areas and turn your text "Powered by PHPBB" into an image so it is harder for worms and nasty people to find PHPBB forums through google.
Also that's for phpbb 2.0.15 which is about 6 months ago. If you are on .15 then you have lots more to worry about that a DDOS.