Create a Forum, or BBS, is easy for now. But to keep it clean and away from spam is more difficult.
To take care of the PHPBB, I use it for G2Soft Supporting Forum, I add some plugins on it.
I added Humanizer mod for PHPBB, and Hide User’s WebSite Button 1.0.3.
I got some help from this two posts:
Preventing SPAM – Bots and Humans
This one is from official PHPBB support team. The advice and steps are very clear and easy to understand, but some installations are very complicated.
Spammers & Scammers
This one is from a geek. He recorded some spammers’ IP address. I use it for reference and ban them in my Forum.
Now, the spam is not that much. About one or two per day. I have to continue fighting against them.