This can only be done when it is your turn.

• Two consecutive passes from you and your opponent. This is the normal way to end a game. Next you both will be asked to mark dead stones. After this, Dragon will score the game and the winner is decided. You should ensure all borders are properly closed before passing.
   
  
• Use the "resign" button. If you resign, the game is lost and your rating will reflect this.
  Resigning a game when it's NOT your turn is also possible.
   
  
• Let the game time out. (But please! Choose one of the other options!) If your game times out you have lost, and your rating will reflect this.
  

In cases of grave dispute or major problems (e.g. bugs or unclear situation caused by the used game rules) a game-admin can also end a game or make it rated/unrated. But please ask an admin only if you think it's absolutely necessary.
In such a case please add a game-ending request in the support forum explaining your wish.