I don't usually visit AC, unless I am on an award F ticket with AA. However, a few UA RCC have terminals, and I have yet to see this problem occur there. If I was approached by a bunch of rude people demanding that I get off of a terminal early, I would tell them to wait. As for the not backing off part, why don't you stand right behind them when they get on the terminal and take notes of what they are typing? That would make a good impression.

I have yet to encounter such rude behavior in the executive lounges of Hyatt and Marriott hotels when people need to wait to use a computer.

Seriously, I have a low tolerance for people who don't have manners. I would be tempted to offer some of those guys a few knuckle sandwiches.