The people I was teaching were kids or adults who were able to vacate an area if they encountered trouble in it, so I didn't even think about that. Guess you would have to be a bit more aggressive/assertive/generally not backing down if you have to guard something.

"Leave now" has longer sounds than "Back off" so it sounds a bit less abrupt, but as you say people aren't rational and they're going to interpret whatever they want to from whatever you say, you could politely say "Excuse me sir, please kindly vacate the premises" in a very calm and completely non-aggressive tone and someone hyped up could still decide to hear you being confrontational.

I'll check out Rory Miller, thanks :)