Mr Angry As long as the player is deemed "in control" of the ball, he is allowed to prevent an opponent from getting to the ball. Being in control can mean that the player isn't physically touching the ball, and its generally recognised that as long as the ball is within 2 metres (6 feet) of the player, then he is in control of it.
^This. There is a pretty loose interpretation of being in control of it tough, 6ft is an awful long way away.