ask him what kind of car he drives... if it's some old beater.. then put the stock cat on your car, and make a bet with him for a shitload of money that your "polluting modifed car" will score better on emmissions test than him. sabatoge his car to make it even worse if you have to lol.. that would probably shut him up. especially if you actually did it...

oh.. and kick him in the crotch while you're at it.. record it and post it too please