In the cases where the law does not provide such protections union will take physical action against the employer.
It is worth noting that it doesn't change the underlying thought I was getting at, though. When a union decides to strike, the options other than "concede" (that's a simplistic way of putting it, obviously, since most strikes result in some sort of compromise) that are available to the employer need to be considered.
https://employment.findlaw.com/wages-and-benefits/labor-stri...
Of course each union contract is different, but it is very common for it to be impossible to fire a bad union employee as the union won't let you. (and in fact the union often won't even let you collect data that shows how bad they are).
None of the above applies to every case. If you don't show up for work, then you are out: no union will defend someone who can't even be bothered to show up. If you do something illegal you are out. These days sexual harassment is something the union won't defend (in the past that wasn't true) and so you will be fired for that. Each contract has different fine print.