Need help blocking spambots in phpbb forum?

I run a phpbb forum and have been constantly getting spambots. I've got like 850 registered users and like 800 of those are spambots. I have image verification on and recently turned on admin activiation but that doesn't stop them from registering. The new bots can't post, but now I'm having a hard time because I still get alot of emails for activation and have no idea which usernames are legit users and which are spambots. And is there a fast way to delete users? Or do I have to manually delete all 800 spambots?