There is a metal black cat sillouette you can get, with a 'marble' for an eye. I don't know how they work, but apparently they do. (my friend who lives in a housing estate used one as she was the only house who didn't have a cat) something to do with the 'eye' but have no idea how it works.
Apparently pepper sprinkled everywhere stops them digging in the soil, but not sure if it works or not.
if all that fails, just get your own cat to chase the others away!
