What might you call someone who verbally harasses a player from the stands?
Hey Lincecum. I've seen better curves on a square!
Who is that guy?
Per Wikipedia heckler is still the proper term.
If it is heckling between players, we call this trash talking. (Or Wikipedia suggests sledging when it is between cricket teams.)
The other term for this guy is jerk (or any variety of nastier nicknames).