I have an apple tree in my garden and there is a cat doing its business under it at least once a day. Its not my cat but I don't want to harm it. I've tried pepper and orange peel. but its still coming back
If you see the cat doing it's business, try firing a water pistol at it. Cats do not like water. A more desperate measure I read about is to get some lion dung (yes, it can be bought) from your nearest zoo and spread it over the offending. Trust me, you won't see the cat for dust.
Call animal control, borrow a humane trap. When you catch the cat, turn it over to A.C. There are likely leash laws against free roaming cats. If the irresponsible owner cares about the cat, they can retrieve
it from impound. If not, it will be found a new home.