Lay standard chicken wire down on the surface. Plants can usually grow in between the gaps, but cats will find the texture of the wiring unpleasant to walk on. If the plants need additional room to grow, you can use wire cutters to cut larger holes directly above the plants needing more room.
If a cat has already marked its territory in your garden, you might need to wash its scent away before it will consider going anywhere else. Spraying or pouring ordinary white vinegar around an area that a cat has attempted to claim can discourage it from using that spot again.
Use an ultrasonic device to deter cats. These emit a high-frequency sound that cats detest, but which humans cannot hear. These devices are motion-activated, so that when a cat walks by, it is startled by the high-frequency sound and should leave the area. --