See the 'FAQ' link on the bottom left side of the home page. It has answers to many questions. In a nutshell, there is a link provided to delete a game below your game board but only during the first 2 moves of a game. After that, you would just have to let the game timeout, resign, or offer a draw and hope that your opponent accepts. I'm sure that you could NOT delete a tournament game. If it's a multi-game match, I don't know how that would work. Here is a cut-and-paste about deleting games from the FAQ section:
"You can delete a started game until both players made two moves only. It means that after the player who has not started the game (black player) performs his/her second move, nobody can delete this game anymore. The main purpose of this feature is to allow players to get rid of a game where the opponent did not start to play at all without waiting for a timeout.
If your game can be deleted, i.e. it fulfills the previous conditions, a link "Delete game" will be added to the bottom of the game page, just nearby "Offer draw | Resign" links.
A deleted game is not counted for anything. Statistics of both players (number of won, drawn and lost games) will stay unchanged and the game will not be calculated for the players' BKR."