Maybe adding a question to the registration like
"Which game is this forum about?"
"What month comes after march?"
"What is two times three?"
"Enter 'I am not a spambot' into this field."
"What came first the chicken or the egg?" (Both answers are working
I think it's called Q&A CAPTCHA and should be easy to install. Also using reCAPTCHA should block more spambots than the phpBB 3 CAPTCHA.
Some more information about spam protection in phpBB 3:http://www.phpbb.com/community/viewtopic.php?f=46&t=2122696http://www.clickonf5.org/6926/more-spam-protection-phpbb-forums-extra-fields/