We had an ongoing prob with moggies in the front garden, the dog used to nearly always keep them out of the back. Especially when we've put in plant dug the earth etc, they thought it was there own personal toilet. If you crush up eggshells [leave in a warm oven overnight] they don't seem to like it and also scattering used coffee grounds seem to have driven them elsewhere thank goodness ;+)
we have one of those electronic ultrasonic cat scarers which works very well. Get one with a mains adapter because they eat batteries. The only problem we had was with one cat who turned out to be deaf.