Nothing is quite as annoying as this! I understand your frustration completely. You've tried the basic ideas. A few other things you might consider are sprinkling baby powder--apparently cats dislike getting this on their noses. I frequently use this black netting I bought at a garden center to keep birds from eating ripe fruit on trees. I cut it into smaller pieces and anchor it to the ground with landscape pins or staples (got those at a home center). The netting comes in a giant amount, and I just cut it to the size I need. You don't want to pin it super tightly to the ground, but leave it sort of loose, so it would present a potential entanglement problem to the cats. They'll actually stay away from it, but you might snag a few birds on occasion. I use that to keep baby bunnies out, and it works great. I also use rubber snakes from the dollar store with pretty good success. You just have to move them around or the animals figure out they're not alive! Another consideration is rock, as you have said, or you could look at www.gardeners.com, Gardener's Supply Company. They have all sorts of innovative ideas for various pests. You could check with your city ordinances, but you may have the legal right to live-trap the cats and take them to the shelter. That could start a neighborhood war, though, so I would be very wary to try that. It's a good technique for strays. I'm sorry you're suffering through this. I know it's maddening.