There should be a rule like: if you work for a private company, you can have a union, but if you work for a government, you cannot have a union. Maybe adjust the details as necessary, but the idea is that there should be no police unions etc.

The reason is that when a union in a private company goes too far, that company can be replaced by a competitor. But if you cannot similarly replace a police force, nothing prevents a police union from going too far.