Thanks for writing! Cats can be a tough one to deter; when they're determined, they scan be stubborn.
That said, there are a number of sprays you can try --- I've heard from other readers that they have varying levels of effectiveness. One you might try is Cat Stopper: http://www.messinawildlife.com/products/cat-repellent.php
If sprays don't work well for you you might try laying chicken wire over the soil in the areas the cats prefer to use. The chicken wire will prevent them from scratching the soil --- and often forces them to go elsewhere.
If you often see them while you're out in the garden, I've also heard of gardeners having good success keeping a watering hose or watergun nearby. When they see cats in the yard they spray water at them. After a few dousings, many cats decide to go elsewhere.
Good luck! I hope this helps!
---Justin, Senior Garden Editor, BHG.com