Cease and desist letter, certified mail, return receipt.
http://creditboards.com/forums/ is a good resource.

Green Tree calls from this number upwards of 6-8 times a day. They have been told that althought I know the person their looking for, that person is a neighbor and I am not their personal answering service. The neighbor used my home phone one time to call these people because their was disconnected and now I'm the one being harassed none stop. And it does no good to tell them to stop calling because the won't. The very rude lady told me the account holder had to tell the to stop calling that number.

Green Tree Servicing

This number has called my home several times.