Knowing that you've lost is a skill that comes with practice
I agree, but as evidenced in the three comments above you, it is a piece of etiquette that is yet to be adopted, even by many of the top players. (Just to make it clear though, falcon isn't a top player. The nicest thing I would call him would be "mammal", but there is still the possibility that he is a highly-developed insect or reptile.)
The biggest issue here is that there is nothing you can say to people who have clearly lost. And I mean OBVIOUSLY lost. You try to explain to them with maths that they can no longer win. If they think they can still win at that point, it is an insult to my intelligence. But they still get offended, even though they are the ones at fault.
There is unfair advantage to be gained by dragging out games beyond their natural term, as noted by the knuckle-draggers above. Especially on the ladder. So you need to be careful with that.
@powderpuff - would you not mind if, in every single game you ever played, every opponent fought to elimination? You agree then, that there is a line to be drawn.
And don't give me that prescriptivist shit like "they can do it if they like, it's their right". You're also able to boot people 1 second after the timer hits zero, but it's considered bad etiquette because it makes the game suck and wastes everyone's time.
I'm approaching retirement (sorry qi) anyway, so I don't know why I even care. I'm just trying to make the game better for you, the humble and honest forum-reader. No no, there's no need to thank me.