Buy yourself a cheap fence charger, and string a wire about cat high on insulated posts around your yard.
If anything like what I experienced, you'll hear an even more awful squawl one time (per cat), possibly a dent in whatever is nearby, never have that cat in your yard again. (Cats can learn very fast when it is in their best interest.)
No, it doesn't kill them, but it does train them that elsewhere is a better place to be.
Depending on the charger, it will kill rats, though.

Cheers,