can a fan interfere with a foul ball
Such interference often occurs when a spectator in the first row of seats reaches onto the field to attempt to grab a fair or foul fly ball. If the umpire judges that the fielder could have caught the ball over the field (i.e., the ball would have not crossed over the plane of the wall), he will rule the batter out on spectator interference. Also, the spectator who commits interference is usually ejected from the stadium. Note that spectators are allowed to catch a ball that is in play when the ball has broken the plane of the spectators' side of the wall. The area where both fielders and spectators are legally allowed to catch the ball is colloquially called no man's land.
Interference (baseball)