Get a dog.
Call animal control, maybe they will let you borrow some cat trap cages so you can catch 'em and bring 'em to the humane society....
Isn't the most common solution to put cayenne pepper extract /flakes into the topsoil to discourage the cats from digging?
Ive heard you can use something like chicken wire flat on soil with cut outs for vegetables. That they don't like to walk on it and they don't like to mess where they can't dig.
To catch a cat all you need is an open box:
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Motion sensor sprinkler