Answer: You might try some of the spray-on or granular form cat repellent products sold at garden centers. Please read the label to make sure they are safe around children, and also apply and reapply them per the directions for best results. Another option might be the motion activated garden hose sprayer -- it will squirt the cat with water.
Somtimes cats prefer one mulch over others, so changing your mulch material might help. Some gardeners report success using thorny branches (eg rose prunings) spread on the ground to deter the cats. Others have used wire mesh such as chicken wire. These make it hard for the cat to dig.
Finally, you might talk to the cat's owner and see if they would consider keeping it indoors. Indoor cats are safer than outdoor cats which can be hit by cars or killed by dogs, so this would not be an unreasonable suggestion. You could also check your local ordinances and see if there is a leash law for cats.
As a last resort, you might select one area such as a far corner and make it very attractive to the cat by placing sand there. It might prefer that to the rest of your yard and at least that way you would have only the one specific area to avoid/clean up.
I'm sorry you are having such a problem.
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