Cat's don't like the smell of oranges, but don't use them as you will attract ants and bugs.
I find dried orange peel works as a deterrant if a cat comes on to your property.
If you tried the suggested, mustard on the bum.... chances are high that the cat would rip you to shreds whilst trying
I don't suggest anything that would be downright cruel!
If the cat has a collar... it means it most likely has an owner, so maybe try and follow it and speak with the owner.
Alternatively, if it's a hungry cat and eating from your own cats bowl.... then change the feeding spot of your cat, then the other cat will either find another food source,... or go home.