I'm surprised at the 'joking' cruelty expressed by some posters here. But maybe it's useful; I'll know who not to talk to, and who to keep away from.

Jimlad, the first thing to do is to clear your garden of cat shit, because it's obviously currently recognised by cats as an ok place to defecate.

(Usually this means an area that isn't any other cat's territory.)

Keep it clear, and water well so that the inviting smell dissipates. Scatter some cayenne wherever there had been shit, to cover up the smell.

Now, if you don't mind a little work, get chicken wire and put it down just under the soil. Cats cover their shit, and they dislike catching their claws in this wire.

The ScareCrow sounds great.

That page of statistics didn't open for me, by the way, Bee.

Jimlad, while the cats using your garden as a toilet may be the local pusses, they may also be feral. If you have a population of feral cats, you could call your local humane society or SPCA, and with any luck they'll capture and neuter the cats, gradually putting an end to the problem.