This is a very difficult problem, and the answer depends on how determined that cat is. For example, a cat that lives in an apartment around which there is only a small patch of garden nearby to act as a litter box, that cat will be very determined to poop in it. Some things you can try are: # place chicken wire on any bare soil so the cat can't scratch it up. # place chicken wire or bird netting over the tops of all the plants, draping it down to the ground in a single flower bed. # spray citrus oil on the area where the cat has been pooping. # scatter citris peels around the area. # place a motion-activated sprinkler in the garden. # keep a squirt bottle handy and squirt the cat any time you see it in the garden. # make sure the cat has a clean litterbox available in a quiet spot that isn't near any other strong smells. Stop faffing around get a Jack Russel terrier.