Cats hate citrus (open an orange and let them smell it and they'll take off running). Spray orange oil or citra solve on the soil around your plants (if orange oil, dilute or it will be too strong for you).

You'll have to experiment with how often. Obviously it won't hurt the plants if a little got on them, but I am unsure how much on the actual plant is safe for the plant. We are going to have to spray some outside plants here where I live to keep the kitties from pooping in the beds.
Jenn