I also have nothing against cats, but agree that cleaning up after an animal that isn't yours is a bit much. I have suffered from this problem for a number of years, since my next door neighbour moved in with three cats, all of whom seem to regard the gravel outside my lounge window, and by my garage, as their own giant litter tray. First things first - remove any droppings, and disinfect the area thoroughly. Then spread some moth balls around - a little uslightly, but they do work, remember to replace them every few weeks. If that fails, hang a few strips of cooking foil where the wind can blow them - apparently cats hate the sound. One or both of these solutions should work. best of luck!