Yep...a nice, powerful pellet gun makes for a permanent solution. If that is not possible, laying down deer netting and planting directly into it can help...if they cannot dig effectively, they won't squat there. They also don't like walking on the stuff.
There was a neighbor's cat using the underside of my parent's porch and wheelchair ramp for a potty. I cleaned it out, spread some huge mulch nuggets--the pieces of wood were about 4 to 5 inches long, an inch or two wide. Too big for a cat to enjoy scratching around. Then I sprinkled hot pepper flakes liberally in the area, some cheap stuff that I bought in bulk. The combination of the wood chunks and the hot pepper worked.
I am a cat lover, I had a stray I found for 19 years. Unless cat is a working animal ( mice control) they belong indoors. Older woman in neighborhood started feeding a stray who happened to be a female. If you thought rabbits multiply, cats are a close 2nd. Nature has a way of controlling things, just when it was getting out of hand, coyotes. moved in
Since I just spent $150 on three cats that are not mine, I don't agree that "the perfect cat is the one who finds you" [me]. Or rather I would assert that I wasn't found so much as abused as a grandmother.
I'm not much for cats, so @Smart Red , I can relate. Both cats we've had found us. Reluctantly, I gave in to the kids begging. They were both nice cats though, lucky for them.
I placed chicken wire around the blueberries, so the cats made their deposits just an inch or two away from the wire on harder ground. Also, ON the crushed rock walkway through my backyard. I have a fairly large animal live trap that I have used for over 30 years . Caught everything from rats, to cottontails, possum, raccoon,small dogs, cats, etc. . I set this trap in various locations in my yard for the last 3 weeks with taste dry and wet cat food, tuna fish, raw hamburger ... still nothing has been traped. At 4:30 this morning, I heard cats fighting in our back yard, as soon as I carefully opened the back sliding door armed with a pellet gun, 3 cats scrambled over the back fence. GONE !!! After the sun came up I found Newly deposited cat poop on the chicken wire. Hey cats ... the wood lot has flufy leaf litter that is MUCH EASIER TO DIG AND MAKE YOUR DEPOSITS IN !!!