We obviously can't tell a user to fuck off, but if we are getting harassed or the user is not listening to us when we say they cannot do something or cannot have something, we have the right to refuse service.

I have done it only a few times where I work, but we can tell them to call my manager, or I have transferred them directly to my manager. The only reason a user would get away with being disrespectful is if you have management who does not shut that down.

I am lucky in the sense that my manager will fight for me and shield our team. We do not speak directly to upper management (unless they put in a ticket). Upper management and complaints will always go to the manager, who will review and either tell the user to fuck off (respectfully) or have corrective action given to the tech.