Personally, I think the kitty litter box outside in your garden would be a great idea, too. Since my past gardens have always had problems with rodents and bugs, I would have been delighted to have some feline garden patrol. And if they hunt and eat there, they won't poop there.

Cats are extremely intelligent creatures, and they aren't gonna fall for the pennies in a can or the squirt gun trick. They'll just learn to avoid YOU and go on about doing what they want to. I love dogs, but they are dumb enough to fall for that trick every time.

The cayenne pepper and associated suggestions are extremely cruel, since cats have no way of getting rid of the burning sensation, they aren't adapted to handling hot foods like us, and water will only make it worse.

I understand your frustrations though, as my kitty LOVES plants. There's a concoction available here called Bitter Apple, which is a foul tasting, foul smelling spray. I finally tried it out of desperation. (Last time I bought a bottle for a chewing dog, my dog ate the bottle.) It worked like a charm. Kitty took one smell and won't go anywhere near my plants, although I do catch her staring fondly at my giant palm from time to time. I don't even spray the plants anymore (it washes off with water.) If you spray your garden perimeter, maybe they'll stay out?