What can you put on grass to keep cats from using it as a bathroom?

Spiky wire or larger Cat Poo...

https://www.wikihow.com/Stop-Cats-From-Using-Your-Yard-As-a-Litterbox

A dog.

We used a large container of cayanne pepper in a 10' x 6' area. We sprinkled it over the area to cover the ground with a fine but even coat of the pepper. We had one more visit from the cats then that was it. It smarts them I'm sure but it is not poisonus and doesn't cause any long term pain. It keeps them from coming back..

Put the cayenne pepper powder where they frequent the most to start out, watch where they go and put in the other spots and so on, eventually they will learn that your yard is horrible and will leave it alone. Just keep an eye out then, in case one tries to come back.