Oh I know UncleUB, I used to suffer from that as well. I bought a couple of them sonic cat deterrent gizmos a couple of years ago. They do work. If I leave them with dead batteries for more than a day they are back. The ones I have have a silent to human hearing mode. This is paramount otherwise it drives the neighbours balmy. If it just involved the ones with the nuisance cats I would leave it set to human hearing mode.
I don't blame the cats, I blame the owners that open the door on a morning and shut the cats out all day long. Sometimes they just leave them out all night as well. You can see them on a morning, sat on garages, sheds, fences and such (the cats that is) just waiting to go for some more food.
If these owners knew where their 'pets' had been then they most certainly would not have them on their lap etc.