Would you mind having another pet?
http://happycarpenter.blogs.com/the_happy_carpenter/golden_eagle-thumb.jpg
Or another cat:
http://www.savannahcats.com/images/new_pa6.jpg
Get something mighty, get something carnivorous, and more importantly, get something territorial. The eagle will kill and eat, but only average a cat every couple days. The Savanna cat is fast, lean, and agressive toward other cats all the time. He will fuck them up on a more dynamic level; he see's, he attacks....or he fucks. If he knocks up a cat, be sure to get the litter, it'll be worth some coin. Raise him around people and he won't be disfuntional like ours.
Point: The only way to fight nature is with nature. Find a lawn fertilizer that is a skin irritant, or just cover your boarders with urushi oil. The bad things about these, and land mines, you cant tune them to effect only the cats, they effect all ife forms, even you. Get an animal to chase them off.