I try to avoid reacting if I can help it, but it ain't always easy. I'll often have a little rant to myself under my breath but that tends to be about it. Like everyone, the severity of the "crime" will tend to decide which way I'm starting to go and then the attitude of the other party will finish it off .. we all make mistakes* and if there's no harm done and people apologise that's cool; hopefully they'll learn and having salt rubbed into the wound helps nobody.
But if - like ChrisKH bloke - they take the "you don't pay tax, I'm more important than you 'cos I spent loads on my car" line and seem to think it's somehow their right to do whatever the f*** they want and everyone else can be damned and die, that's a different matter. Although I agree with you, Chris, I doubt you'll ever change the mind of someone like that: they KNOW they're right and everyone else is wrong, with their attitude of "end of, FACT !!!111". Also, those ones tend to be the ones that will try to smear you along a wall if you wind them up. <sweeping generalisation mode off>
* Edit: I know I do. And once or twice people in cars have been patient with me when I've cods'd things up on the bike.