| As was stated in How to fight and not get your ass kicked, I'd like to maintain that getting in a fight is bad. If you can, you should talk it out. But sometimes rational discussion doesn't work. Sometimes you can't just settle things by thinking them through rationally. What can you do? Talk big? Stand up straight? Look tough? Hardly, you'll only provoke the fight. My advice is to remain calm, you don't have to look like you're in a good mood about somebody wanting to take you on, but don't play it like you're eager either.
Start out by telling this person that you don't want to fight. Say that fighting isn't worth it, and it doesn't accomplish much more than putting otherwise decent people into the orthopaedic section of the hospital. Nobody wants that. If the person continues to be aggresive, then you're gonna have to resort to a physical threat.
Now, you have to make the threat sound logical. It has to be something they could easily envision, something they could see you doing, and it has to bring home just how serious they've made the situation. Given the proper threat, you can stop a crowd of people intent on beating your ass. My favorite threat to use is the eye.
You can probably adapt this to other body parts as well, but I'm not sure if I could come up with a better threat than this (in fact, I didn't even come up with this, one of my friends did. I've only tried to get out of one fight with it, I don't usually have to resort to it, but it did work.). So... here goes.
"If that's the way you want it, you can fight me. I don't want to do this, I'm just reminding you, but if you even so much as take a swing at me, I'll take out one of your eyes. You may beat my ass, put me in the hospital, or even kill me, but remember that if anything comes to harm on your body, the very least damage you will suffer will be the permanent loss of your depth perception. How would you feel about being called 'lefty' the rest of your life, wearing a patch, or having an exposed scar, and being the butt of bad pirate jokes till the end of your miserable existence? Do you think you'll still come off as 'handsome' to the ladies with an eye missing out of its socket? I doubt it, but c'mon then. You want to lose an eye?"
I hope nobody who reads this ever has to use it, but it works in lighter matters too, and can keep you from being thrown into pools at partys, because it works on groups by the fact that you can throw the element of randomness in it.
i.e. "One of you is going to lose an eye."
Well, I guess that's the extent of my advice, have fun and don't start fights!
Unless the dirty punk is asking for it. | 
