You can cancel an active Classic game that's waiting for your turn from the list of games on the main screen. To do so, swipe the game you want to quit, to the left, and you will see the option to “Resign”.
If the game is on hold, you can click on it and select “Resign“.
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 impacted.