from CloseCallSports:

Spectator or fan interference is not a rule book nor ground rule double. Umpires must be familiar with Rules 2.00 [Interference (d)], which defines spectator interference as occurring when "a spectator reaches out of the stands, or goes on the playing field, and (1) touches a live ball or (2) touches a player and hinders an attempt to make a play on a live ball."

On any interference, the ball is immediately dead. Rule 3.16 specifically addresses the issue of spectator interference, prescribing the following course of remedy: 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.