You will be able to cancel a Classic game that has been started by selecting the box with a white flag, located at the bottom left-hand corner of the screen. Once you do, the game will ask you if you would like to resign and you must press "Resign" in order to end the game.
Keep in mind that this will negatively affect your statistics since the game will be considered as lost. That is why, if you receive a request for a game in which you would not like to participate, we recommend that you reject it before it begins. This way, your statistics will not be affected.