ULPT Request: How to deter stray cats from using garden as litter box 💩

I used to enjoy decompressing in the garden and now I dread it.

The smell of cat piss is nauseating and to add insult to injury I'm allergic (to cats)

This is now a personal vendetta.

How do I deter their behavior before resorting to a bb gun and Elmer Fudding the situation.