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One of the best solutions is a motion-detecting sprinkler system that are gaining popularity nationwide just for this and other 'pest' reasons (skunks, moles, opossums, etc).
Another solution is add some unfriendly plants to the areas the cats frequent. Depending on the part of country you're in, these could be anything from cactus to fragrant ground cover (reportedly they are put off by the pleasant smell ?).
A solution that worked for me not just with cats but with all other mammals is a hefty sprinkling of very hot pepper (ground red pepper, chili pepper, cayenne, even black pepper). It deters mammals while allowing for birds (who are virtually immune to hot pepper effects and don't perceive capsaicin like mammals do). The problem with this is that it can get costly since you need to replace it or refresh it every few days depending on wind, rain and sprinklers.
Certain fertilizers (other animal dung) put them off too, but it's likely to put you off as well.
Lining the ground with chicken wire until the cat's habits change and they just stop coming by will make it difficult for them to get a 'dig' going and they don't care for the feel on their feet.
If you have a sprinkler timing system (or want to get one), setting it to go off at or around the times you normally see them might dissuade them.
No matter what you do, just remain humane about it. These animals are doing what they're supposed to do. Just acting on instinct and by nature. The offending animals are the humans who own them and let them loose. That's ignorance and it's not only dangerous for the cats, but one day they may bring their owners' home some really bothersome disease, worm or fungus to share with the entire household.
I appreciate that you're asking this question rather than just reacting - it shows you've got lots of patience and ethics!