I wouldn’t start by firing them, but by fining their union a daily fine until it obeys. Remember that rank and file have little say about what the union bosses choose to do. But if the union bosses tell them to strike and they refuse, they *will* be punished.
So the solution is fines. And if the union refuses to pay those fines, it is contempt of court, and the union’s officers can be put in jail, as well as placing union property in receivership.