There's another relatively recent discussion of this issue, and you'll probably find others if you use the search at the bottom of the main forum page with the term 'cats.'

Here is a link that might be useful: Cat business

Here's a bit of an update to my original thread. I still haven't made it to the garden center to pick up any of the suggested products, but I did try the citrus with possibly some success.

I can't say for certain yet if it works, but the areas of my mulch I've sprinkled with chopped up lemon and orange peel have remained free of poo. Before treatment, the cats were doing their business everywhere on my mulch. The areas I haven't treated yet have daily piles. Maybe it's a coincidence, but I'll continue to chop up my citrus as I use it. I guess I just need to use more lemons and eat more oranges.

Kevin

Cayenne pepper works for me. My neighbor's cat used to love to use my front bed as a potty. Since I've been liberally sprinkling the cayenne, he never visits any more.

Subject recently discussed in a lengthy thread -- board search function will find it if it isn't on the first page or two of the forum.

Grandma sticks plastic forks in her planting beds with the forks sticking up, but tries to disguise the forks with some mulch. Cat stays away.

Another option would be to bury mothballs underneath the mulch. Keeps cats out of houseplants too if you bury it an inch below the surface.