While I do see what you're saying, the girls were galloping their horses in a pasture that is intended for warm ups and for watching the events. From what the OP posted, the girls' behavior and disregard for anyone else is dangerous.
I would talk to the barn manager/even planner about the situation, and see if there is any rule or something that can be implemented to prevent this type of behavior. It's just not safe, IMO.
The barn manager didn't do anything. He was freaking out though b/c a horse that was boarding at his barn a couple years ago slipped in the pasture b/c he was running and broke his leg and had to be put down. But he didn't say anything.
Maybe you should talk to him about that. And explain to him/her that you think this was a really dangerous situation. And unsafe for other riders as well. And maybe make a rule, like 'No galloping unless you're in the arena' Or something?
That's a really good idea, but the barn manager is leaving this July and they're going to hire someone else. I bet the someone else hasn't gone to any of the barn shows and hasn't seen those stupid girls.
I'm not saying just sit there and not do anything. I'm just saying that you CAN'T do much without a barn manager or the parents of those girls backing you up. You can tell them they are stupid. You can tell them they are going to hurt their horses, you can even accuse them of not caring about their horses, but you can not physical stop their behavior w/out back-up from somebody with authority. That is all I'm saying.
And going up to someone and throwing out insults regarding their behavior by telling them they are stupid, is only going to make an already immature person behave that much worse. I.e. Galloping past you at a close distance.
There are ways to handle people like that and it takes experience dealing w/ that type of personality. But they bottom line is... they are responsible for their actions. Be mad and get loud but that is the bottom line.