Best thing is to try & keep your head cool. I've had a few "close encounters" myself, but I always back off in time... If someone starts acting like an idiot, I simply keep a distance. One piece of advice: never ever get out of your car, then it goes wrong for sure, one way or the other.
About a month ago, I just had my alfa for about a week, I was driving with a friend (in seperate cars) on the highway, and we were talking using our hands-free-set. He's just 18, with little driving experience, and some arrogant beemer driver didn't like that my friend passed him with his dad's "grandpa" V40 volvo, (but which has about 180bhp).
The bmw was just changing lanes to overtake a truck and didn's see my friend coming, and he didn't use his indicators, so my friend used his horn. The bmw was p1ssed off by that, and tried to take over my friend from the right (illegal manouvre here in Belgium) after he agressively changed lanes and ended up between us 2. My friend is still young didn't want to back off and started doing reckless stuff. With 180bhp under the hood, he easily prevented the overtake (which wasn't the smartest move imho), which for sure surprised the other driver. What happened next was a sequence of harasments and stupid things in both directions, while I tried to calm down my friend.
I'm pritty sure an accident would have happened if I wasn't on the phone with him at that moment. He got so upset and mad, I tried to calm him down, and got myself in my brand-new car between him and the other hot-head. I got some distance between those 2 and told (actually ordered is a better word
) my friend to back-off and drive behind me, leaving the other car in front of me. The bmw did try to slow down to get to my friend, but I didn't let him (in a rather friendly way). Then he got angry at me and started working it out on me. Luckily for me - he was a worse driver than I am (not saying here that I'm the best driver in the world), stayed calm and easily evaded his "tricks", keeping a good distance. It ended in me taking the first exit (actually a petrol station) at the last moment while he was in front of me - not being able to follow me and my m8... I was really surprised by my friend, he usually is a very calm guy but what I heard there in the car I almost couldn't believe.
What the BMW driver did was inexcusable, but my friends reaction wasn't the best either. I sure did some crazy stuff too there, but I felt it was necessary to keep him away from my friend, and I only evaded the other guy's manouvres without much trouble, which was p1ssing the other guy off even more so I ended it.
Just try to keep calm - it's no use, if the other guy wants to act like an idiot behind the wheel - just don't give him the chance. Sure it's tempting to show him what you're made off - but what's the use? Is that guy really worth it? I don't think so... I rather be the one with the brains (and undamaged car/body