I know AA has identified and banned people for hidden city ticketing ... but accompanying them to the gate?!? Seems outrageous and a waste of time given there is no world in which AA can legally force someone to board a plane. What examples/evidence of this actually happening is there?
There's at least one thread somewhere on the AA forum detailing this. Of course, no-one was actually forced to board a plane, but essentially the conversation was along the lines of "we know you've missed this flight lots of times so we think you might need help finding the next gate. If you don't, here's the amount you owe us for the total of the fare differences you've evaded". So of course there was a choice, it was just between a rock and a hard place.
BA is, as you can see from the advice here, somewhat more relaxed about this.