Originally Posted by flyerCO

You'll be in jail for false imprisonment if you try that. No crime against you has been committed. No crime against DL has occurred yet either. Unlike a law enforcement officer, you don't get immunity if you detain someone and it was incorrect. The only exception to this rule is for merchants and their employees who do get immunity. Even then it is very specific circumstances that they get it.

You likely have a civil case to sue DL, but no criminal case against the "thief". You don't own the seat, nor has DL leased you the seat. DL could file charges for trespassing if OP refuses to get off plane after being asked. (If they refused to move) If passenger moves back to C+ inflight then they could also possibly file theft of service charges.