If you have or use one of these, you're a jerk. 

A plane in the US had to be diverted and two passengers removed after one of them started a fight by using a banned device to stop the seat in front reclining.
The spat began on United Airlines flight 1462 because one passenger was using the Knee Defender, a $21.95 lock that attaches to a tray table and jams the reclining mechanism of the seat in front.

All major US airlines prohibit the use of these sinister and selfish devices so why this person felt the need to disobey crew instructions is beyond me. 

The FAA can impose a civil fine of up to $25,000 for passengers who are unruly.

Sadly no one was arrested or fined in this instance but it's only a matter of time.