Ok, it sounds mean, but have you tried a scat mat? It's a grey mat that gives off an static electric shock when touched. I have one that I originally got to keep a certain feline member of my family off the kitchen cupboards (where he likes to pee!!!! ). It's very effective for the cat and has the added bonus of keeping the dogs from counter surfing too. The shock doesn't hurt--I've accidently been shocked a couple times. It just startles them more than anything.

I got mine from Pet Supply House--a Canadian site in Canadian dollars.