I have suffered with this problem for years, one house alone opposite me having four cats, all wanting to use my gravelled garden as their toilet!
Someone told me a couple of years ago to try mothballs, which I've been doing, dropping them roughly every square metre or so all over the gravel. It definitely seems to work but only as long as the mothballs are still quite pungent - you'll need to refresh them every 7-10 days, but they are only �1 a packet in Wilkinsons!
I also received a tip yesterday which I haven't tried yet, this was to spray white wine or cider vinegar around the perimeter of your garden. I also heard or read some while back that cats can get scared if they see themselves reflected in a mirror, the idea being to leave small mirrors dotted around your garden - not sure if I want that look though!