[calmbonsai]

This is only true for the "ticket purchase" not for the service itself.

Outside of ERs in exigent circumstances, any commercial enterprise in the U.S. retains the right to "refuse service" though the nuances of enumerated reasons backed by jurisprudence differ by industry and locale.

An airline can not refuse the "service purchase" unless the customer has been "banned" (technically a trespass statute), but they can refuse to "execute service" for a whole host of reasons including unforeseeable "Acts of God", logistics, or simply if the customer is intoxicated.

[ehasbrouck]

This is not true, in at least 2 respects: (1) a common carrier has a legal duty not just to sell a ticket but to provide transportation according to the tariff, and (2) this means an airline can't order a passenger not to board, or order them off the plane, unless they have violated some terms of the tariff. Some airlines have tried to create their own no-fly lists, but without opening up another area of discussion these have no more basis in law than the government's no-fly list, and have never (so far as I know) been reviewed by courts.

[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.