I have tried to implement the following, but I could not make it work.
Allow registrations, and the user must get a mail with an activation link. Newley regestered users, group.
Designated moderator group to approve all posts made by newly registered users.
When a new user posts a spammy message, the moderator group to kill the message and the account.
When a new user has posted 5 non-spammy messages, he's whitelisted and further postings require no approval.
I did a new user as a test user but I could not get the posts to show up in the moderation queue.
Maybe other users with more phpBB experience can help?
Might also try: http://stopforumspam.com/ https://gist.github.com/deltreey/797970
We could also going from Q&A to reCaptcha?
PS, User registration settings has been set back to: disable new registrations. But I am not sure this is working ether as spam users seem to be get in....