Opposition players can cause frustration by deliberately blocking you from getting to the ball. Players grounded / standing on the wrong side of a ruck is an example of this.
What to do...
Any opposition player preventing access to a space next the ball can be cleaned out! Simply move your forward onto the same space as the blocking player. Now move the blocking player to any empty space next to him.
Note: A player can be 'Cleaned Out!' and moved into a 'Protection Space' resulting in the cleaned out player being rucked.