Cats don't like Jeys Fluid - I put it around my front door to stop unwanted spraying. You could put it around your cat flap and other places where the cat is getting in your garden (it may stop yours as well, perhaps you will have to let them in and out for a while). You can buy it in a spray bottle from supermarkets - it doesn't smell too bad! You need to apply it every few days, especially if it has rained. My Mum had a magnetic flap and had to remove it because the cats kept getting their paws trapped.