Cats are the most obstinate of pets to train, and their markings and urine are repugnant. I had 11 cats as a child, which ran around outdoors around my Ohio home. They were affectionate but independent, doing whatever they pleased. (At least we had no mice.) When I asked a few knowledgeable professionals your question, I realized it wasn't going to be easy to answer. Veterinarians were careful to point out that whatever you do should not hurt the animals. They do prescribe medications to control cats' desire to frequently urinate, which may partially solve your problem, but since these are not your cats, the owner would have to make the decision. Trappers suggested cages, distasteful foods, etc. Pepper, garlic, and other chemicals all were possibilities, but they may also have an effect on you. The most common first answer was to get a dog. Perhaps your yard is not well suited for a dog, or there are allergies in your family, so the second most common answer was to fence your yard in. Cats are apparently attracted to your yard. It occurred to me that you may have a problem with the shrubbery in your yard, which may create an odor similar to cat urine. The boxwood shrub, which has small leaves in clusters, smells something like cats when the shrub's leaves are wet. Crazy but true. The best answers I found for you are to spray water with a mister at the cats until they get the message that they are not wanted in your yard. The spray makes a hissing sound they fear, and they hate water. You can also order something called Jerry Baker's Dog and Cat Repellent. It advertises that it keeps dogs and cats from using your lawn as a "dumping ground." You simply sprinkle it on the areas you want cat-free and "your troubles are over." It costs $8.95, and you can order it by calling (800) 355-0559. Comtrad Industries sells the "Catscram" device for $39.95. It's an electronic device that uses passive infrared motion detection and ultrasonic sound waves to keep cats away. The device works over a 6-foot-wide area at floor level or a 15-foot radius when elevated, using a 9-volt radio battery. A word of caution: Avoid using chlorine bleach to clean the urine from anything on which a cat has left his mark. The combination of the bleach and ammonia can create a toxic gas that has been known to overcome the user, and can be fatal. Good luck!Readers are encouraged to send questions to: HERALD Homes, 379 Central Avenue, P.O. Box 220, Lawrence, N.Y. 11559-0220Attn: Monte Leeper, architect.