I agree totally with Parabellum on this. This situation should never have got to the aircraft, it should have been sorted at the gate, like all such incidents should be. What were the gate Staff doing at the time of boarding?
As an initial opening bid when dealing with pax, try treating them with tact, consideration & respect, when this has no positive responce, then possibly call for the cavalry. I would agree though, that in some circumstances, the odd pax does not deserve such consideration.