UK/North America language thing here, but by "garden", you mean a patch of lawn rather than a veggie or flower bed, right?
I only ask because I find it so weird that cats are going on your lawn. Cats tend to be quite private about poop and like to bury it.
I'm surprised by all the reports of cat poop on lawns/decks/etc.
I actually assumed that's why there aren't laws about picking up after cats the way there are about dogs. I always thought that it is because it's simply not as much of an issue in it being in high use areas.
I'd try the cheapest deterrant suggestions, then go from there.
I just had to clean up the most disgusting mess when my two year old stepped in dog poo at the park and tracked it all through the house before we realized.
Not cool. So, I feel your pain!
Also, I'm a big cat lover, but I agree - cats are an introduced species. They have been domesticated for 4000 years or so, but revert to feral very quickly largely due to their brain design - they have a flared brain case, with more brain area concentrated near the brain stem - the more primal area where instinct, reflex, etc is governed. This does not mean that they can't be conditioned to stay indoors. You have to be really vigilant, though.
Statistically speaking, they live a lot longer when they are strictly indoor cats. And they don't wreak havoc on the local wildlife, either. Mousing is one thing, but migratory birds take a huge hit from domestic cats.
Just some food for thought.