phpBB3 is pretty solid. Try it out, as it requires the user to enter the image text which bots cannot see. This almost eliminates bot registration. Of course there are always ways in and close monitoring is the key. The problem then lies in the fact that actual humans might register and then let the bot take over posting.
I am starting a project to fight spam, as well as most other types of fraudulent internet activities. Included will be a forum which I will have to make super secure. I will let you know how it goes and if I pick up any additional tricks along the way I will pass them on. The website will be
www.internetcops.net
There is not much there now, I just registered it the other day and made a quick Coming Soon image. If you or anyone else would like to participate I would love the assistance!