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There are a few methods of preventing cats from defecating in yards. Most involve using plants or scents that discourage the cat from entering the yard or garden as can be found here: http://www.petfinder.com/cats/cat-problems/how-to-stop-a-cat-from-using-the-garden-as-a-litter-box/
You may also want to provide the cats with a litter box or clear place to deficate in the area that you want them to go. You can do this by moving their feces to the location you want them to go. It should be a place with soft soil that is easy to dig in.
Generally cats will bury their stool in areas closer to their main home area, but not when they are out hunting so providing them with a clear space may help.