Have you lodged unpaid item disputes against them?  If not you should do so immediately, not only will you get your fees back and be able to relist the items, but you help other sellers by giving the buyer a strilke.  If all sellers did this, these buyers would not be able to repeatedly do this. Once they receive 2 strikes against them they will find the number of sellers with whom they can deal is severely reduced.  Follow the UID procedure, and make sure your blocks are set to restrict buyers with strikes.