Enzyme-based odor neutralizer can be sprayed as a cat repellent on outside doors so that cats do not return to refresh their urine spray scent, according to About.com. There are commercial repellents available, as well as devices that detect motion and make a quick, loud sound to drive feral cats away.
Reducing clutter and making digging difficult may keep stray cats at bay. The smell of rue is unpleasant for cats, so planting it in problematic areas can be beneficial, according to About.com. Citrus peels are also disagreeable to cats, and thorn bushes or other prickly cuttings can keep stray cats away naturally.
There are organizations that can help remove stray cats humanely by trapping, spaying or neutering, vaccinating and eartipping them. They are then released back into the wild or given up for adoption. Although this may not always be an immediate solution, it does reduce reproduction, eventually bringing the stray cat population down.
Stray cats choose to live in locations where there is food and shelter available. Keeping outdoor areas clear can remove their motivation. If there are any smaller prey such as mice in the area, it is important to get rid of them, which in turn eliminates the stray cat's food source.