Finding a humane way to chase cats out of your garden can be a real problem once they've already started using it for a litter box. A friend of mine had this problem last summer, the ferret cats in her neighborhood was using the flower pots on your front porch and front yard as their litter box and the smell was so embarrassing for her when someone would come to visit and walk down her front walk. Even the mailman would cover his nose.
I have to admit, I'm a cat lover and I feel so sorry for the homeless cats and the cats that people throw out once they stop being a cute kitten. It's such a shame that so many people won't go to the expense of getting their cat fixed and keep them indoors.
There are diffent things you can do to try to scare them away. Like cutting an old garden hose and laying it through your garden so hopefully they think it's a snake. You can also plant Rue around your garden, cats hate rue, but at this point it's probably too late to grow rue. If you can find someone that is growing rue you can get some leaves off of them and throw the leaves around your garden.
I think right now you need to spray the area they are digging in and using as their toilet with a mixture I found in a book by Dr. Myles Bader. I gave this recipe to my friend and it finally helped chase the ferret cats away from her flowers and front porch.
Mix together in 2 quarts of warm water:
2 tablespoons of cayenne pepper
3 tablespoons of powdered Chinese mustard
4 tablespooons of all purpose flour
Let stand for at least an hour and strain. Add mixture to a spray bottle and spray the area they are using heavy with the mixture. You might have to do this for several days but it will finally chase them away to find a new liter box.