I happen to be a spectator at a high school game over the weekend. Played under FED rules.
It was the 2nd or 3rd inning and I hadn't noticed any fan comments directed at the umpire to that point. On a pitch that appeared pretty far inside the umpire called strike 3. Many fans started yelling things like "The was a ball" and "Come on Blue", and "That was a terrible call". I didn't hear any profanity, and thought to myself, that did look like a bad call, but figured it would die down as soon as the next pitch was thrown. All of a sudden the umpire turns to the crowd and says "Your out of here". Everyone just set there for a moment and then one fan says who are you talking to, to which the umpire of course says "you". Then another fan says you don't even know who said anything. The umpire then says "your out of here too". A school official came and made them leave the field.
My question is, what do you guys that have a lot of experience, use as a criteria for throwing out a fan, and does the FED rule book or case book give guidance on this?