I have the same problem, most mornings my car hood looks like they were doing the cha cha on it all night long. When I wash it I have to stand guard to keep them off till its dry. I thought about a car cover and then I thought what if they still get on it, putting their claws into the fabric, scratching my car. Also, what if they do decide to sharpen their claws on the cover! They certainly sharpen on car tires when they feel like it. So, I have not been able to find a solution myself. Spray them with a water bottle till they learn maybe.
Can you believe they actually make fake paw prints decals to put on your car?! We get em for free.