you can buy stuff to put on your garden to stop cats from doing that.
I have some can't think of the name off hand. I got it from the garden section in the hardware store.
there are other way's as well.
Trish
You name it - I've tried it. From chemicals, evil smelling liquids, movement actuated ultra sonic devices, mannequin cats with glowing eyes, pepper dust, lion shit and other things I have long forgotten. None have deterred the cats - the only effective device is my catapult with painful but harmless plastic shot. But even I cannot sit up all night to await the evil visitor. The other problem is that my wife is a cat lover - hence my options are severely restricted.