You can easily see that a player is F1-ing out of a battle or even out of a chase. It gets in a slide, regardless of direction, does not fire, nudge or even wink... and then suddenly POP he left the server... now... you can do 2 things... kill the pilot so when he gets back online he is dead OR screen him a couple of times while he is going in a straight line, then he pops out and screen the departure message too... Its hard to prove, but a few nice follow up screens should prove you point.
"War. War never changes. Since the dawn of human kind, when our ancestors first discovered the killing power of rock and bone, blood has been spilled in the name of everything, from God to justice to simple, psychotic rage."