How can a keep stray cats from pooping in my flower bed?

I have stray cats in my neighborhood, along with skunks, possum and who knows what else.

My two dogs chase them away if they are seen, but the cats usually do their business at night.

One of my dogs even had to have emergency surgery as she ingested some rocks that a cat had pooped on. (I've since removed as many of the rocks as possible). Both dogs are always searching the flower beds for the stray cat poop.

I've purchased a motion sprinkler that covers most of the yard, but the flower beds closer to the house don't get covered by the motion sprinkler, nor do I want it blasting the side of the house.

Is there something I can put in the dirt, such as cayenne pepper that will discourage the cats from pooping, yet won't hurt my dogs? I've read cayenne pepper is safe for dogs, just not sure how to apply it. Does anyone have a mixture that's used, or do you just sprinkle the cayenne dry spice around?

If you have other suggestions that might work better, I'm all ears.

Thanks!