A water pistol is a very effective deterrent for a cat, or buy the Cat Poo Off crystals you can get from places like Homebase, and sprinkle them round the area where they poo, Crystals are more effective than sprays. They smell gross but they keep the cats off. Mothballs have a similar effect. You can also buy Lion's Roar pellets - remarkably, they kid the cat that you have a lion, and they don't like that.
I had a similar problem when my neighbour cleared his garden, the cats had pooed in it for years while it was wasteland. I have now set up a litter tray in my garden which at least reduces the problem.