You know who these guys are. They do not weigh 400 pounds and have no choice due to their natural body configuration. They sit next to you on the airplane. They probably got to their seat before you specifically so they can stake out as much space as possible. They view the flight as a game of Risk, only they're staking out the territory with elbows, shoulders, knees and feet. The space they paid for ends at the mid-line of the arm rest, not two inches into my seat.
And short of physical violence, there is nothing you can do to get them back north of the 38th parallel. And no, shoulder spreader, arm rest stealer dude, I was not trying to pull a Larry Craig on the airplane, I just wanted to be able to sit in my seat without having the right hand side of my body jacked.