Actually...it is quite legal to shoot cats as they are classed as vermin. As long as you do it on your own property, or a place you have permission to shoot on, you can't get into any trouble. The only thing you can be charged with is destroying private property, and this only stands up if it is on the owners land...so blast away boys
Cats cannot metabolise aspirin-feed it to them and they die from internal bleeding. A slightly less drastic method of discouraging them is a supasoaker water gun, filled with half and half water and hot chilli sauce-when they loick themselves clean they get quite a shock
A simple plan: get a really big, black dog. I used to have one and watch it "scramble QRA" after the cats that came into the garden to shit. Not a very clever dog, though - once it managed to race to the fence completely missing the cat, and stood there whilst I jumped around like an utter c-nut, yelling and pointing. The cat looked around and quietly walked off.