I am not condoning using physical force but what would you do?
Full flight and a person with a boarding pass for 74E is sitting in your allocated seat of 40C becaused it looked better when they were boarding. Your boarding pass has 40C. Their boarding pass shows 74E. Crew refuse to get involved. Do you get off and go on next flight? Do you keep your mouth shut and sit in 74E for this long haul flight? What if you're travelling with someone? Tough luck?
I struggle to comprehend the advice. Don't be a vigilante because it's not the right thing to do but please go and sit where you don't belong because people will feel sorry for you and let you sit there? Where do you draw the line? Should I go and sit in a First Class seat and see if the crew force me out? Or is that going too far? Are we saying there is no difference between 40C and 74E?
It has happened to me a few times. Crew has managed to sort out the situation. Just as well as my anger management is poor at best of times.
As
Pushka says - and has been mentioned upthread - you can't take 'the law' into your own hands.
You cannot act in a way which is threatening, intimidating, or interferes with crew or crew duties. You must also follow lawful crew member instructions. In this case, it is likely a crew member trying to secure the cabin for departure will be issuing a lawful instruction if they direct you to another seat as it directly relates to the safety and operation of the aircraft.
No one is suggesting the seat poacher is in the right, or their actions are to be condoned. But you have very few options on the plane to remedy the situation if the crew are unwilling or unable to assist you. Your only real option is to disembark, but you'd want to have pretty substantial and solid grounds before making that decision.
The time for you to seek redress will be either in-flight (perhaps the CSM can offer immediate compensation, or prepare a report for compensation at a later time), or after you land, via customer care.
Is there a difference between 40C and 74E? That depends. If you have paid extra for 40C (extra-legroom) you have a contractual right to have the money refunded. If you haven't paid for that specific seat, your rights are pretty much to be transported from A->B, in the cabin you paid for.