Are they fouling on grass or on earth ? If on earth, has it recently been dug over ? (they love this, it's their freshly laid litter tray !).
You might wish to put some lemon or orange halves around the garden, I'm told that cats hate the citrus smell. Chilli powder might also help - but obviously both these would need to be renewed at regular intervals.
Water pistols ? Have you got a good aim ? PS I live with two cats but they have their own toilet in my garden and don't make messes in anyone else's, thank goodness.