In the house I've heard of people putting crinkly paper or foil on potted plants to keep cats out, but in the garden that probably wouldn't work. If you can get a motion detector that would turn on a sprinkler or make a noise (there are quite a few animal repellents that work that way) or there is a product I found online called "cat scram." I suppose it works along the line of rabbit and all of the other "scram" products.
Scram for Cats Repellent - maybe someone will come along with another suggestion, but you might want to look for reviews of this products. Since the website says it is a mix of aromatic plant oils, you could also try your own mix to see what would chase them off. I'd start with red cedar chips and see if that smell isn't strong enough to deter them from using the garden.
My cats were house cats so I don't have much to offer as far as that experience. Every spring it seems after I dig up the beds one of the neighbor cats decides to try it out, but for whatever reason, maybe because I put down wood chips after I finish working on beds, after a time or two they move on and leave it alone.