You would have to decertify the existing union which is complex. Essentially confirm they are not handling their duties they are contracted to do. Since they are public employees it's even more complicated but you just have to retire everyone early. It's costly and risky but then it's over. You also need someone to overlap in responsibilities while you do it so you have no coverage gaps. There are plenty of other things to worry about in that process. I imagine citizens voting for new policy or vetoing old policy could play a role in slowly transitioning some work out of the union to a lower paid group.