Don't know if it works, but apparently citrus peel can deterr cats, also a short length of hosepipe, they think it's a snake.
Prickly leaves is good to, not that you really want to leave them on your lawn, but you could try a few temporarily to see if it works.
The water pistol is a good idea, you don't want to hurt the cats as that could get you into a lot of trouble.
Carol