Because commercial repellents haven't worked for you, consider physical barriers, such as mats of flexible plastic spikes or buried plastic forks (tines sticking up). High-tech (and high-priced) cat-deterring methods include high-frequency sound emitters, some of which are triggered by motion sensors. Finally, if you can't beat 'em, devote an area of your garden to them. In a corner of your garden plant, catnip (Nepeta cataria) or catmint (N. x faassenii) and install a sandbox for the kitties. The cats will find the plants irresistible, and they'll prefer to dig in the sandbox rather than the heavier soil in your garden. These distractions may occupy the cats' attention enough to keep them from exploring the rest of the garden.