I had the same problem before I bought a house with a garage. I used this:
CatStop.
It seemed to do the trick. I angled it in a way that it covered the entrance to my carport, and the paw prints and cat hairs never came back. This wont work on really old cats because they don't have such great hearing, but it works like a charm otherwise. The only downer is that it gets set off by any motion, so it goes through batteries pretty fast. I just figured it was a small price to pay to protect my paint...