help for a friend who's having problems with strays in her garden

HI everyone.... long time no post... is there any advice that you can give to a friend who is having problems with the neighbourhood cats who are using her garden as a cat litter box? Is there any natural way to keep them out of her garden? I've heard that lemon peel deters cats from using it... any help?

They say any kind of citrus peel will work. Lemon, orange, lime or grapefruit. Also I have heard that moth balls also keep them away. My neighbor who is an avid gardener always plants a couple rows of marigolds because apparently cats don't like the smell of marigolds..(don't blame them they do smell horrible.)

Hope those suggestions help.

We are using something called "Critter Ridder" in our garage. We have a ground squirrel infestation.

Here's the link. It's supposed to work on roaming cats too, but is non toxic.

chili flakes sprinkled over the ground works but that hurts them.

So we bought ultrasonic movement detectors. they sound like a little click to us and the cats HATE them. it cleared our yard of the strays TOTALLY. and Abi got used to it when I assured her the first few times she heard it from outside.