I'm with FF on his perspective that this is a much as part of the national pastime as is the game of baseball itself. I may give it to them every inning all game long. But I will always, always make it a point to go up to them after the game and tell them they called a great game if they were dead on. It goes both ways for me. I've had some that I was riding and they would turn around and give it back to me. They gained my respect at that point and I let them do their job.
What's crossing the line? I suppose cursing, swearing, threats, obscenity, etc. I'm not very shy so if I ever saw that I most likely would go up and approach them and strike up a conversatoin with them to help them calm down. Knowing my luck...as Im standing there trying to calm him down and Blue makes a bad call I may let him have it with a "COME ON BLUE!" Lol.
YGD