I think given the situation you describe, I'd start with something basic and concrete. You do mention that you've kept telling her to stop, but is it maybe "too nicely", or just as an interjection, where maybe she doesn't take it seriously(?) If so, maybe stop to dead silence and firmly say something like, "Please do not call me your BFF. It is not accurate and I don't like it."

Make space around your statement. Don't be apologetic. Be very firm and crystal clear. Eye contact.

Or is that how you've already told her?