If you have games that you do not want to play, go to the "my games" page at the top left side of the page and click that. You will see a list of your current games. Look to the right side of that page and you will see a trash can icon by any game that has less than three moves completed.
Click the trash can icon and the game will be deleted with no change to anyone's statistics.
If you do not want anyone to challenge you, go to "my settings" at the top of the page and then go to 'challenge settings'. Set a limit and then click 'update ' at the bottom of that page. If you set it to zero, you can still challenge others or create 'open invite' games, but you can no longer be challenged by someone else.
EDIT: oops, too slow. Trev beat me to it...