If an airline has the attitude that you have to pay for a seat regardless of who/what is spilling into it, an unofficial check-in policy might be helpful - perhaps all passengers who look like they may not be effectively bounded by the armrests (and any particularly rude ones) could be assigned to a particular row?
Row 13 maybe, or - more sensibly - somewhere near the CofG for that aircraft type.
Any "overspill" could therefore be counteracted from the adjacent seat.
Pity about the last person in the queue when the only seat left is 13E in a 10-wide layout...
