I always get questions about a sort of reverse RentBoy.com.In fact, if you look on some of the message boards older posts, you can see it's a topic many escorts have questions about.
One such place that comes up on a www.Google.com search for client blacklist is this website, http://www.blacklistednow.blogspot.com.
It seems to be a Virginia, Maryland, Washington DC area list, but there are phone numbers from all over.
One of the biggest sites that you have pay for is http://www.nationalblacklist.com and looks a bit more commercial to me.
