Shame, get a laser pen and shine it on and through their curtains every time they go out when it's nice and dark. The cats will wreck the room, flower pots over and the blinds will be **** as well. It's like ruining his life but from the very core of his self, everyday. Eventually your patience will be repaid and you may see the silhouette or silhouettes of sobbing people in the front room, you know the up and down shoulder action when they realize Poppy the cat has used Aunt Mable's ashes as impromptu litter tray or just tut next time you see him on the street, if he appears not to hear you then run after him "Tutting" in his ear loudly, but not in an overbearing rude manner. Neighbor relations are so very complex I find, I feel for you. I live on a boat now.