There is no doubt that being a moderator is a thankless job. Hubby moderates a site dedicated to one of his hobbies and after they began having similar problems they made a few changes. First any new sign up must be approved by two moderators. Sometimes it is easy to spot a problem just by the handle they use. Secondly for the first month being a member every post goes to a moderator for approval before it is released to show up on the public forum. It is a lot of extra work some days but it has pretty much eliminated the problem.