if the crew just went and got a pushback clearance then the airport car by law would have has to move as he is now obstructing a aircraft which has a clearance on a taxiway.
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.
Before anyone else starts banging on about this being a safety issue, it is not. AS PIC if I want or need to take my jet in there I will. Then I will walk away and let my company deal with the airport baron.