How can I stop cats from pooing in my garden!?

I have 3 dogs and I was told that by letting them do there biz in the garden it will stop the cats coming in and pooping. It hasn't worked. I do pick up straight after my dogs have done there biz though. But the cats keep coming in and there killing my gr they don't even bother hiding there poo! Lazy!! Has anyone got any tips apart from getting a cat! Not a cat person really and my dogs would probably squish one if we did! We are trying leaving bottles of water around as aparently they don't like it and those high pitch things that are supposed to deter them doesn't work either. I'm picking up about 10 poo's a day and my garden is only small! Help