Water is a good one. Used to have a cat come into the back garden and attack our cat. Kept a water pistol by the back door, it soon got the message. Best to shoot through the window as they often hear a door opening. Our own cat has also become much better behaved now aswell......
As an aside I was waiting in the car outside and the cat jumped onto the bonnet, expecting to be let in I just stared it out until it got off. THEN I remembered the windscreen washers[:'(]. Gutted. The stupid animal then jumped onto the boot, giving me a second chance. Wasn't remotely bothered by the rear wash wipe, didn't expect the electric arial tho! he he he. Animal wasn't harmed, just got off, sharpish[]