When I played Shoddy and when I clearly lost, I just tell them "Well, I got nothing left" and say gg and leave after they respond.
Most competitive games and competitive players actually follow this practice in an effort to "save the other guy's time".
In Pro starcraft leagues into this, when a player decides that he cannot come back, they say gg and leave. In fact, when they broadcast the league games on TV, the announcers use "gg" as the end of the game. The other player don't want to waste the other guy's time. The player signals that he has lost, and quits the game (surrenders)
In another competitive game, Puyo Puyo, most competitive players kill themselves when they know they lost assuming they're not on some major event.
Even the winning player, when they see the other player has lost, will refrain from triggering their chain to save time
Another example is Chess, as X-Act brought up, has the option of resigning and surrendering.
Now unless I'm mistaken, the match isn't over until 6 pokes have fainted, correct? Why are there so many people that play and leave as soon as it's clear the battle's not going their way? Am I the only one who thinks this is quite bad sportsmanship, or do others get annoyed by it?
I guess some people prioritize the event of the game itself (So that they get the "full satisfaction of finishing them off") so they must see all 6 pokemon die and some people just prioritize their time (so they surrender and leave to save time) and resign and leave.
Sadly discussing Etiquette is rather redundant since the people who discuss it are the people who follow it anyway and "douches" don't normally bother reading through this kind of stuff.
Of course if you're implying people who decides that they cannot win, signals that they have lost and leaves, is a douche, it's another issue.