Cats have an aversion to citrus. I have heard several have some success at keeping cats out of the gardens, by spreading a few orange peels, or lemon peels around. I haven't tried it - don't have a much landscaping worthy of protecting any more - but my neighbor does. They have two outside cats, and they have been doing this for over a year now, and swear by it. They put about five oranges worth of peels out once a week or so.

Cayenne and black pepper will work, but wash away easy, where as the citrus peels will get stronger smelling when wet.

I would stay away from moth balls. Many people use them and say they work great, however, they are very toxic to cats and dogs.