Originally Posted by Spiff

In the case of the person who is not a POS occupying your seat:

Since you're now talking about passenger behavior, rather than passenger dimensions, a word to the FA would probably result in the passenger who is intentionally intruding into your space being asked to cease doing so, with continued willful intrusion being constituted as failure to follow airline crew directions and therefore subject to arrest.

I suppose that after the flight was completed in such a case, you could also attempt to file a civil suit against such a passenger who had truly made your flight a miserable experience but I don't know how well that would play out, even in small claims court. [note: I am not an attorney, so please do not consider this to be legal advice.]

As for a POS occupying part of your seat:

You need to speak up before the flight and proceed as I posted earlier. If you do not, then you will likely be deemed to have accepted having part of your seat occupied and the airline will do nothing for you after the flight is completed. The POS owes you nothing either.