Deleting a multi-player-game by the players is not possible at the moment.
 
Reasoning: One player shouldn't "destroy" a game, that all agreed upon.
If the player does not want to play, he should say so before the game is started or refuse from joining in the first place.
Maybe this policy will be changed later.
 
Please ask an admin in the support forum to delete a multi-player-game if the need arises.