Toots, you're right: pepper spray and other "cat repellents" are usually not very effective. They wash away quickly, and even if you continually reapply them, eventually the cats will get used to them and just ignore them. There are some plants that may do a better job of detracting cats, and there are also ultrasonic devices you can put up in your garden to emit a sound that's very unpleasant to cats, but inaudible to people. Jonathan goes over both of these and some other solutions that may work for you in his article on how to get rid of cats.