Ignoring the Civil Aviation regulations for the moment, is the position not this? I'm curious to hear other thoughts.

The plane, including the space where a passenger is sitting, is the property of the airline.

The passenger may have airline's consent to be on its property, perhaps within the terms and conditions of the passenger's contract with the airline. The airline can always revoke its consent. The nature of the passenger's right to occupy the property is barely even a license, let alone a lease.

At its highest, the airline might breach the contract by revoking its consent - the passengers contract (being the ticket and the associated terms) might give the passenger contractual 'rights' to remain in his or her seat and to be conveyed to a destination. This is probably not the case, and it also assumes that other terms do not permit the passenger to be removed.

One way or another, the airline can revoke its consent, even if it means the airline breaches the contract. The passenger might be entitled to damages, but I am sure there are terms excluding liability, unless other legislation applies. Specific performance is not available because damages are not an inadequate remedy, so that is the only remedy available.

Once it does revoke its consent, gives notice, and the passenger fails to leave within a reasonable time, it is entitled to use reasonable force to remove the passenger from its property.