There are a number of battery-operated deterrents on the market which, when switched on, pick up movement with an infra-red detector and then emit ultrasonic, high-pitched frequencies that reputedly scare off the culprits. They are not audible to humans and the usual range is about 10m (33ft) over an arc of 70 degrees.
Another solution is to put down citrus peelings or lemon scent where the cats are causing the damage, or prickly prunings around their favourite plants.
They are less likely to use your garden as a litter tray if they have no access to bare soil - especially dry, loose soil. If you have a gravelled area, try replacing it with larger stone chippings or pebbles.
Unwanted CDs can be threaded on twine with knots in between to keep them apart. String these across flower beds or hang from trees, and the light reflections will deter cats.
Try growing the annual Coleus Canina, labelled as the 'Scaredy Cat Plant', a pretty blue-flowered annual which smells foul to cats. It's widely available online.