Cats are territorial, and like to 'mark out' an area they take a fancy to. Deterring them is twofold.
First, and most importantr, you need to remove the scent markings they leave being. If the area is paved, scrub it thoroughly with strongly disinfected hot water, rinse, and repeat. The sprinkle moth balls around the borders, paying attention to an ariel access - walls, rooves and so on.
Hopefully the need for cats to re-establish their dominancec, which they do by covering any other cat's spary with their own, will be nullified by removal of all odours, and the moth balls should discourage them from feeling the need to start again.
Be patient - repeat the proceedure once a week, and ensure the moth balls are replaced when they melt away, and you should find your visitors going elsewhere.