Is there something I can use to stop cats from leaving ';droppings'; in my yard?

I have a patch of dirt in my front yard where it seems some bushes or plants were and removed before I moved in. And for some reason, cats have been leaving droppings there.Is there something I can use to stop cats from leaving ';droppings'; in my yard?
Its very hard to stop cats once they have used a certain spot, you will have to remove the dirt, cover it or plant lots of flowers so they can't get to the soil.Is there something I can use to stop cats from leaving ';droppings'; in my yard?
I have a neighbor who tries to save cats and has around 18 of them. (Grossly irresponsible!)





Twice these feral critters have given birth in my front bushes! They use my garden in the back yard as their litter box. I tried talking with her, but she is psycho. These cats are not domesticated! No pretty little kitties!





They are mean and nasty, and have all sorts of diseases. You haven't lived until you hear a collective group of them screwing at 3-4am. You'd swear the end was near.





I started growing catnip everywhere...that drove them nuts. I got a dog. I peppered them with paintballs.





The country considers them vermin and will not come pick them up. They told me to do what you want with them. I've had to do things to the birth kittens and mothers I don't want to repeat here. It was a terrible shame, but I can't have cats giving birth by my front door and then attacking anyone/everyone who comes to the door. Getting sued isn't my idea of fun. I say do what you have to do to rid yourself of them.
I had the same problem with my neighbor's 9 cats using my potted plants as their liter box. I got a product called Repell at the pet store and sprayed a liquid fence around my yard and it worked well to keep them out. You can also try sprinkling cayene pepper on the specific spot that you are seeking to protect. It wouldn't work for me because I have automatic sprinklers that go off every morning and couldn't be vigilant about reapplication. But, cayene pepper does deter them.
Get a dog.
A 22:)
Small cliber pistol or high powered bb gun!