Make them pay for two seats. If they’re in a middle seat, make them pay for three seats.
Then let other passengers have those seats for free if they want them, keeping in mind that the fat guy is going to spill over into your seat, invading your personal space, pinning you in awkward positions and stabbing you with his bristling arm hair.
He may even listen to music on his iPod and do a little fat man dance in his — and your — seat, wobbling around like fat hairy jello.
But you’re flying for free! You still want it?