Try and catch them pooing and spray them with a water pistol. Cats learn very quickly and don't like water, if not put twigs over plants to stop them digging. Dulce (neighbours) cat that thinks it lives here goes home (or out) when I pick the water pistol up, don't even have to spray it now and he knows to go home. I know it may be difficult to do a 24 watch with a water pistol but if you catch them once they might go elsewhere. Good luck.