How about, if we get damage, we also get driver fatalities. Have a career mode, where death is a possibility. You could then have online competitions, with realistic penalties, where you could not only loose your cool ride, but your driver as well. I know it's morbid, but it would thin out a respectable amount of wallriding knobs who'd rather bounce off your door than use the brakes. And/or make damage cost money to repair, hell if you drive someone else's car in any weekend event in the real world, you are liable for a decent chunk of change.
Adding cost to damage, and possible death for a career driver character, would go a long way to thin the ranks of bumper car cheats.
Even if it were just a specific level, say call it "Real Mode."