try to cover the soil with heavy river rocks, or sharp gravel. These textures are not pleasant for prying paws, and they can dissuade the cat from turning up the dirt to poop. If putting stones in your plants doesn't work, you can try a cat mat, a flexible mat with small spikes which is actually designed to keep cats out of the garden. The mat won't hurt the cat, but it will make your potted plants an undesirable environment. You can also use skewers in your pots to keep your cats from jumping into them. good luck with that!