[calmbonsai]

Intoxication, ill health, threats to other passengers/crew, assault, and battery are just a few of the many reasons an airline is well within its rights to "refuse service" and this includes not allowing the passenger to ever board the plane.

You're correct that any sort of federal no-fly list is not lawful and, so far, there isn't enough court precedence to make a general policy and the ACLU has won some (limited) court victories. IMO, it is unconstitutional.

In practice, a "banned" passenger is trespassed off of airline property (the plane or gate) and that prior trespass is noted in their customer registry. If the "banned" passenger attempts to fly with the airline again, they are advised verbally or with text that they will not be able to board the plane, but they can still book a ticket. The passenger is then "trespassed" (again) at the gate and not allowed to board.