There are many possibilities and a wide range of opinions on the matter. When I plant a new garden area I scatter fresh orange rinds and usually have no trouble from the neighbors' cats.

We have a small working farm, and must not have cats messing in our vegetable beds planted for the farm stand. Chicken wire works well laid flat on the ground until the newly dug ground has settled. I have also used non-toxic commercial cat repellents, but they are too costly to use except on a limited basis.

Here are a variety of ideas:

http://www.cat-repellant.info/

Several possibilities are mentioned here; 4th post down on Tue, Apr 11, 06 at 18:50. "* ammonia soaked (corncobs, etc) * aluminum foil..."

http://www.gardeningknowhow.com/problems/cat-repellent.htm

http://www.safepetproducts.com/cat-stop-cat-repellent-1-cat-repellent-on-the-market.html