Buy a pump action BB gun pump it 1-2 times and shoot the cat in the rump (not the head) it wont be strong enough to break skin but will hurt like hell. Shoot the cat everytime you see it in the yard then let classical conditioning take its course so that cat makes a negative association with your house and avoids it. If some PETA freak confronts you about shooting a cat pump it an additional 2 times and shoot them in the rump too until they stop coming around as well
