I swear I have have heard that you can buy lion dung to scatter over your lawn. The theory being that no cat will compete with that! The downside of that is having a garden full of lion poop, which is probably no better than what the cats would deposit!
A friend of mine reckons that cats don't like the smell of citrus peel, so she scatters shredded orange peel over her garden. I must admit, I tried it, and I feel I'd have got better results by not peeling the oranges and just throwing them at the little blighters.
I find the best thing to do is try and keep the lawn as short as possible. The cats in my area are choosy about where they perform their ablutions and tend to prefer long grass. Short of lurking in the bushes with a hosepipe or adopting an energetic dog, there's not much else I can think of.