Dont give them anywhere to dig.Make sure you use plenty of ground cover plants.Get some called scardycat. Apparently they hate them. You can get motion sensor water sprays so they learn that they get a soaking. Or buy yourself a water pistol and squirt them when you see them.Not to powerful or you could cause injury.
I have 3 cats myself so I do understand your concerns,that is why I offer safe humane options.
Good luck
be nice to them so you can pick them up then hold them under a garden tap and give them a good soaking. It has worked with one in our garden it now runs a mile if it sees me.
Sprinkle lots and lots of white pepper over where they like to 'go'!! You'll have to reapply after rain but within a few days they get the idea and go elsewhere.
Get some wire coat hangers and cut them into 6 inch lengths. Stick them in the ground with about 3 inches pointing upwards and put them close to each other so that the cats cannot crouch down without sitting on a spike.
No crouch, no Pooh.
You can't even see the wire once it weathers.
I have eventually eliminated cats from my garden with sensors that let off a loud noise (humans cant hear it), but the only problem is they are directional, so if you have a large garden you would need quite a few and would be expensive.
Another successful application is dried holly leaves ( the prickly ones) spread over the area they use. Collect them from your local woods, this works a treat.