How to end a game is a question we get from time to time. Put simply you don't need to end a game. You just stop entering data and it assumes the game is done. You can zero out the clock if you want for a reminder to yourself but it's not necessary. The only number that would change if make the game ended or not is the case where you want to see playing time before the game ends. Version 1.8.8 will allow you to see playing time currently accrued for a game in progress if you are tracking time. Other than that you don't need to mark a game in any way as being done.
This may change as we add some other features which want to know explicitly that the game is still in progress, but that change would be specific to those features.