I put down chicken wire and covered it with a thin layer of mulch. The cats don't like it because they can't scratch without getting their claws caught. I've also heard sprinkling orange peels work, they don't like citrus.
Cats don't like citrus. I've used sliced lemons to keep them out of beds before. Worked great!
I have the same cat issue. Nothing has stopped my intruder so far, except something similar to the chicken wire suggestion. I bought a package a plastic netting I saw in the garden department at Walmart. It’s a giant sheet of very thin, black netting that you would put over berry bushes to keep the birds off them. I spread that out over my front landscaping, cut some hole for my plants to come through, and just anchored it down with little U shaped metal anchors. It’s so light weight that you can’t see it unless you’re looking for it, so it doesn’t detract from the appearance - and so far I haven’t cleaned up any more cat poop!