Can you ammonia to keep cats away from your yard?

What you can do is use the system called Trap-Neuter-Return. What you do is, if the cat is not hissing or spitting at you, let him inside. Keep the cat in your sight while you call the nearest Animal Shelter. Tell them your problem, and that you want to use this system. they will neuter/spay the cat and release it back into the wild once it is well enough. That way you don't have a cat locked up in an Animal Control place when they were just being curious.