Yard

What is the best way to keep my neighbor's cats from using my garden as a litter box?

One downside to creating an area with soft, fluffy soil is that it attracts creatures who like soft, fluffy soil.

You have some options, other than installing a fence tall enough that a cat can't jump it

Use one of the cat-relling prodeucts on the market -- you can find them at pet stores.
Lay chicken wire on the ground where the cat enters the garden, in hopes that the cat won't like to walk on it.
Place an attractive diversion between the garden and the cat's home, such as a catnip plant or a small patch of sand.

Many communities have leash laws banning dogs and cats from romaing roam outdoors. You could report the cat to your city, who will notify the cat's owner. However, confrontations with neighbors can create more problems than the cats does.

Meanwhile, take comfort in the fact that while it might make you shudder to know a cat is using your backyard as a bathroom, it's not doing any harm. The byproducts break down naturally and fairly quickly, just as they do elsewhere in nature. The only time you should be concerned is if a pet is using a contained place, such as a sand box, where a person (child) is playing in the material. I have heard that sea shells in a planter will stop cats from digging in the pots. I think it's like finger nails on a chalk board to the cats. You might try sea shells if you have any available and don't mind what they look like around the edges of your garden.