On transatlantic, my understanding is that the Purser has the option to ask the seat poacher if they'd like to pay the C+ price (typically $99-119) on the spot with their little terminal they use to sell booze (well, sell booze on domestics, since it's free TATL).
Since C+ is a different 'cabin experience' and fare bucket on most routes other than TATL, I'm not sure what options there are other than 'scram' or 'eh, okay' from the FAs.