Yes those cats think the soil in the plant is just like the outside soil. So actually they are in the right. However, it is quite smelly and not a good for you or the plant! So, this will require some intensive cat training. Simply said, the main course of action is to make the soil not seem like the outside soil. So change it somehow that will make the cat think it's not the outside.
Here are some ideas that others have used is:
1. Put rocks or pine cones on top of the soil to make it uncomfortable to get in there
2. Put aluminum foil on the top of the soil
3. Spray the cat with water when you catch them in the act! This one works well for dogs too.
4. Put any kind of screen on top of the soil like chicken wire or other material.
5. Spray a citrus type spray on the top of the soil. Most cats don't like citrus.
6. I am sure there are sprays at the pet store but, Please make sure they are not toxic to the cat or the plant! (last resort)
Good luck with some of these ideas! Let me know how it goes and what works best!- King Oliver