I might try laying down some chicken wire over the preferred places as an
experiment.......

When I moved into my house, the "gardens" were covered with a layer of lava
rock. It was not a deterrent for the cats. They'd just do their business
right on top. I live in a culdesac. I don't have any roaming pets, but
between all the nieghbors, I've counted about ten cats outside and a few
dogs. What's funny is the cats don't bother the birds, and the dogs don't
bother the cats, but boy, it's funny to see the cats and squirrels chase
each other around over fences and up trees.