That sucks. Just ban him!
One thing I've done and I know others are starting to do this as well - create a new member usergroup that is not allowed to post in the classifieds until they reach a certain post count/days registered/reputation level, whatever criteria combo you want. Also create a scheduled task that will bump them up to the next member level once they reach that criteria. It has helped prevent spam and fraud greatly for us. Of course this is assuming you're using vb