Spam is an ongoing issue on all forums, it is notoriously hard to stop the manual spammers which you are seeing now as the automated signups are filtered pretty well.
The best methods to deal with these is via the community, as soon as a post is made it needs to be flagged to the mods/admin and a simple reply that states they are scammers will hopefully stop the foolish from enetering into a deal with them.
We pretty much negate these posts very quickly.
Personally I have never seen a paid membership system work on a forum like the format we operate in, plus the administration side of things can become a nightmare. As a community we can all set about dealing with the issues and protecting our forum.