I have my own cats who seem to find the fine soil and mulch in my garden and flower beds to be the supreme potty area.
I cleaned out the poopy. Then, I lightly sprinkled red pepper flakes around my roses, bell peppers and watermelons. After a couple of applications, it worked. It didn't hurt any of the plants.
When you first put the flakes on the garden, you have to look real hard to see it, and it quickly weathers and blends in, so it isn't an eyesore.
I buy the large container of crushed red pepper from my local wholesale club, so it isn't a huge investment.
I don't remember where I learned this, and I am not sure why it works. My theory is that when the cat goes to sniff the ground to find the primo spot, they get an uncomfortable sensation in their nose. I'd think twice before digging there, too–if I were a kitty that is.