[janaf]

There is the possibility they are manually / properly registerd and no technical system can prevent that, if regestering is allowed. Then there is not much to do then but to ban them when they start postig .

Or if it is a forum with limited number of new members and a specific community, then have everyone introduce themselves writing some specifics related to your forum, in a hidden forum that only admins / moderators see. Then promote or ban them depending on how they write their introduction. Most manual spammers will not bother with this step.