Make her the boss. Let her make all the tough decisions. Let her decide what the band is going to sound like, act like, and present itself. Put her in charge.

Doesn't sound like such good advice? Unfortunately, if you can't talk to her about her problematic behavior you've already done all of the above.

Be nice. Be honest. She's your band mate, so give her the benefit of the doubt; maybe she doesn't see what is obvious to everyone else in the band (Your opinion is shared by other band members, isn't it? You've talked about this? If not, there might some other work you have to do here. You might have to consider your own motives. Just a possibility)Give her a chance to fix what she's doing wrong. She added to the band once, there's no saying she can't again. You might be an important part of her "growth process". Mostly, treat her like you'd want to be treated if it was you screwing up. Next time, it may be.