Possibly, but before in the air would be basically impossible to prove the intent required. Even in the air, would need to show person knew or should've know (ie has in case of a business class seat) that seat required extra payment to sit in.

Basically unless the person moves into the seat inflight and with knowledge/intent of above it's impossible to prosecute a theft of service case.

On the ground before departure,, the really only option is to have removed for trespassing.

In reference to earlier post. As you don't have exclusive control over the seat (unlike a rented house where you have exclusive control of the house) you have no right under law to have the poacher removed for trespassing. Only the airline has the right to demand they be removed from plane. Thus why performing a citizens arrest will only likely result in you being the one charged. Again, unlike a police officer, you don't have immunity if the person sues you for unlawful detainment. You can also be arrested for false imprisonment/unlawful detainment.

IANAL but I can't imagine trying to make a citizen's arrest. It sounds dangerous to attempt, even if TSA airport security makes you confident that the person doesn't have a gun.

The other issue with this (laughable) citizen's arrest scenario is that the laws governing citizen's arrest vary from state to state. Or would this be federal jurisdiction and is so is there a federal citizen's arrest law? And what happens if the seat poaching occurs in another country?

I don't think these situations end up well most of the time when it's done.

I like! Especially because he was stuck in 14B, yes? Easy to keep an eagle eye out and see if either of them got up. Even if 10B got up, I'd snag that seat. And then just keep pointing at 10C saying "Mi asiento" over and over and over...

Possibly, but DL (and its agents) should be the "enforcers".

let us know how this turns out. I am pretty sure you are correct that there will be a much longer delay (for you, at least).

Yeah, tell the gate agent politely but firmly that you WILL be following up with Delta on the matter and he/she can print you a new boarding pass at the computer at the jetway/door of the plane. Get up and follow them to the front door to ensure they do so - the plane wont be going anywhere while you are up at the door (don't exit though...) - if you remain level headed but firm, chances are they might rather deal with the seat poachers than having to deal with you, And to request the police will cause an even LONGER delay anyway.. ridiculous!