M.G.
I'm wondering where this anxiety has come from, has she had an encounter with a boy that made her think they are all mean?
I would have a very grown up type conversation with her. I would just be firm with her that her behavior is unacceptable. You are not going to allow her to be rude to another child (after all how would you feel if little Johnny was treating you this way?, etc.). Some times if Mom drops the "oh, honey I'm so sorry, etc." kiddos will snap out of their funk.
Good Luck,
M