bhilmers wrote:raboof wrote:http://www.phpbb.com/community/viewtopic.php?f=46&t=2122696 suggests a 'Q&A CAPTCHA' works well.
I am the administrator of a phpbb3 forum that was practically destroyed by spam, with literally hundreds of posts per day. The Q&A CAPTCHA solved the problem completely. It was very easy to set up. And the question can be somewhat obvious, like, "what is the word following "music" on the Linux Musicians banner above?" (freely), and still be effective. I definitely recommend the Q&A CAPTCHA.
I second that.
The Q&A 'plugin' should be installed by default in 3.0.10. You can configure it in the 'Spambot Countermesures' bit under the General section. The only thing I have found is that you need to change the question from time to time, as they eventually crack it. And you shouldn't use basic maths questions in the Q&A, as they are easy for spambots to crack.
The normal captcha's are useless - the spambots broke them ages ago.
Another thing you can do (although it involves more moderator work) is to set 'New member post limit' to 1. This will put all new members posts in a moderator queue, so at least if the spammers manage to register their posts won't go though.
PM me if you want any help.