I have a PHPBB forum with around 1000 members but recently have been getting dozens of spammers which are registering daily and posting rubbish on my forum including disgusting photos. I've recently updated it to the latest version 3.0.8 and also using the Re Captcha plugin.
So you must:
1) Fix the hole which is allowing spammers to register accounts by bypassing the captcha form
2) Once I have verified that they are no longer registering on a daily basis, then the next task is to clear all the existing spam accounts on the forum
Please see attached for a screenshot of the problem.