Just don't let the balls occupy the entire board.

In order to win put five or more balls in a line in the direction you like. You can move only one ball to any empty cell. Then if there is a line of five or more balls they disappear, you get points and then you can make another move. The more balls you put in a line the more points you get. If there is no line of five balls, three more appear on empty cells of the gaming board.