Rule 3.16
When there is spectator interference with any thrown or batted ball, the ball shall be dead at the moment of interference and the umpire shall impose such penalties as in his opinion will nullify the act of interference.
APPROVED RULING: If spectator interference clearly prevents a fielder from catching a fly ball, the umpire shall declare the batter out.
Rule 3.16 Comment: There is a difference between a ball which has been thrown or batted into the stands, touching a spectator thereby being out of play even though it rebounds onto the field and a spectator going onto the field or reaching over, under or through a barrier and touching a ball in play or touching or otherwise interfering with a player. In the latter case it is clearly intentional and shall be dealt with as intentional interference as in Rule 3.15. Batter and runners shall be placed where in the umpire’s judgment they would have been had the interference not occurred.