Look, she's in high school so she's still a child. And she's acting like a child.
There is no job, no position in the world, where you will be successful or liked if you behave poorly towards your peers. Unless you're Stalin or something.
This isn't just a running thing, it's a life lesson that she needs to learn. It's not acceptable for her to behave in such a way that she discourages her teammates. School sports are learning opportunities as much as they are athletic endeavors, and it is your job as an educator to make sure she learns to behave in a way that will allow her to become a productive member of society.
I would have a frank talk with her about it. If I were coaching a team and I thought one of the members were damaging it, I might threaten to hold them out of meets. I know it's probably not quantifiable and so you'll have some trouble figuring out when she crosses the line, but my point in general is that I absolutely think it's more important for kids to be successful in their future lives than to race well in high school.