Prevent Cats From Using a Garden As a Litter Box

Plant feline repellent varieties of herbs, such as pennyroyal. Any herb with a pungent aroma will repel them, but pennyroyal particularly offends them.
Lay weedblocker and use rocks to mulch. This will allow you to maintain your annuals, and will frustrate the cats because: rocks hurt their feet and they can't dig to cover.

You could also use mothballs spread evenly around the garden, but be aware that mothballs are made of napthalene, which is toxic to your plants, the cat, you, and everything else alive.