We have two free-range cats, and live rurally. Our neighbours have recently come to us to say that the cats are crapping all over their garden. I feel awful, they are great neighbours and I didn't realise the cats were causing such a mess. We bought sensor activated cat deterrent thingys for them a couple of years ago, and we assumed that they were working, obviously not... 
My plan is to reinstate at least one litter tray in the house, maybe a fancy covered one? In the hope that they will prefer to do it here. We can also limit their outside time as the cat flap can be programmed to lock at certain times.
Our cats have always been free-roaming, hunting cats and any time we've kept them in for vet appointments etc. they've been literally climbing the walls. They would be extremely unhappy if we had to keep them indoors all the time. Is there anything else I can do? Cat-proofing the garden would be difficult, and I feel that I wouldn't be able to give them enough outdoor space to keep them content.
I completely understand why my neighbours are unhappy, they don't even have cats and should be able to enjoy their garden without finding cat poo! I'd love to hear some success stories please.