|Howdy, long time lurker, infrequent poster here *waves*
Is there any way you can refine the registration process? Can you make it so you can ask some questions of the member trying to sign up, based on what they answer can determine whether or not you approve the membership. If they are spam memberships hopefully (in my experience) the questions will weed them out. Then you can deny them before they even get the chance to post on the board.
I'm not sure if your Admin Control panel will allow it, but I thought I'd mention it anyway?