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.