Sure they can. It's pretty simple really. In a 7 game series, the winner is determined by who wins the most games. In this case there are 7 games. If the Warriors win 4 of the 7, the most the Spurs could win is 3. Warriors 4 games plus the Spurs 3 games yields a total of 7 games played. Since the number 4 is greater than the number 3, The Warriors would have won more games, thereby winning the series in 7 games.

Hope this helped.