I have the same problem and can't find an answer. Have tried all sorts - sonic deterrants, lion poo, plants that cats are supposed to run a mile from (as if!) - water pistol is a deterrent if you can lie in wait for them, because no matter how quietly you open a door to go out and squirt them, they're off like a shot. We have an open plan front lawn with an area of shrubbery on one corner that pre-dates our occupancy. Cats spray regularly on this and it smells disgusting. DH has tried spraying Jeyes Fluid on it, but it doesn't make a ha'porth of difference. Removing it is not an option.