How would David Cameron stop the Royal Mail strike?

He told Gordon Brown to "stop the strike", and accused him of having no back bone. Since Mr Cameron has spent several years telling us how Labour has eroded our freedoms over the last 12 years, what exactly would he do to end the strike? As far as I can see, postal workers are free to hold a strike ballot, and to go on strike as they see fit, their choice. What terror tactics does Cameron have in mind? Images of Thatcher's Stasi clubbing miners spring to mind.