How to stop cats from using my garden as a bathroom

Cats do not like moth balls. If you put a box of them in your garden area they will cease to want to go there. When the moth balls have dissolved (evaporated?) they may be out of the habit of marking their territory in your garden, or you may need to put out another box for the summer to break their habit.

https://www.thespruce.com/cat-repellents-to-keep-cats-out-of-your-yard-2132573

I‘ve heard criticism from some, of course, but find cayenne or tabasco to be fairly effective. My Grandfather used it. Yes, it would be washed away w/rain, so go sprinkle more. You can purchase a large size reasonably at Sam’s or a dollar store. My Grandfather said it was the smell that kept pests at bay, after the initial introduction. They don’t come in my yard at all unless they’re newbies. It’s even been effective w/deer & other wildlife that like to feast on my bulbs or flowers. My theory is that, I’d rather use a more natural approach. I’m not a fan of the chemicals. We have a dog, as do most of my neighbors, but they typically don’t Go in my flower beds. Cat urine definitly will kill plants. They also have great noses. Good luck.

Pit moth balls in it they will not harm your plants

Cayenne peeper or chili powder sprinkled in the garden. One sniff and they'll remember. May need to reapply.