Poor Pussies, I hope you wouldnt shoot them?! One of my cats was shot my and air riffle and the vet said the safest option was to leave the bullet in.
At least they have good taste choosing a Z to take a nap on.
Most garden centres do cat repellant sprinkles, so you what it says on the tin and sprinkle them about the place you dont want the cats. (prehaps not your car!)
As Ekona mentioned, ground pepper works well. I use this as some moggie is keen on piddling on my wheels.
I have 3 beautifil kitties and i am allergic, Wouldnt be without them though!
Hope it works!