. I think you will find that the law is a little more complex than that. The airport baron knew what he was doing by directing staff to block the aircraft. You assume a taxi clearance confers some legal right to remove the aircraft from the airport ramp.
As much as I am not a fan of the airport there is possibly another side to this. Just perhaps, Canberra Airport has being trying unsuccessfully to negotiate an agreement with QF and in their often heavy handed way QF have refused to negotiate, leaving the airport lease holder with a problem and they saw an opportunity here to apply some pressure.
I agree there would definately be a little more to this than the original story. On saying that though even if QF wasn't paying their bills you still can't just go and block aircraft. You would have to turn up with some sort of legal covenant preventing movement on condition that the bill is paid. It's just the same as anyone else who owes money. And with Qantas you know where the head office is it's not like they're going to runaway never to be seen again.
Really it is not something that should be dealt at a operational level, this is commercial law stuff.