Well, first you would need some indicator for when this condition should be checked. This could be a time limit or a limit on number of moves. It could be at a certain break point as like the end of a level or something else. Whatever this indicator is, when indicated, check the score and either allow the game to continue or not. To stop the game, usually all it takes is a "Game Over" text message (with possibly showing the score) and stopping the scenario (using "Greenfoot.stop();").