Grrr, I HATE irresponsible
pet owners! NO pet should be allowed to roam. Too many cat owners are under the illusion that their little kitties don't really leave their yards, or bother the neighbors or harm the native wildlife.
Rent or purchase a trap and catch them. Return them to their owners, ONCE, and see what happens. Explain that their cats are a major nuisance and are soiling and causing destruction in your garden.
If the cats return (their owners have made the choice for you), trap 'em again and then either take them to the vet or....
Some people also have luck with sprinkling cayenne pepper around on the ground beneath their disturbed flowers/veggies. The pepper will burn the cats' mouths when they lick their feet. They'll know this is not good and might equate it with your garden.
Some people also place plastic or metal forks, handles buried down, into the same areas where cats are a problem. Their hind ends and feet don't take kindly to sharp spikes!
Of course, these last to options can cause problems for your
children and pets, too.
If you are in the city limits, check and see if there are statutes regarding roaming pets. Perhaps being cited by the law and being fined could curtail the problem.