If you have the authority to reprimand, fire, or otherwise discipline her, I'd send her a formal note with copies to the partners and/or HR if there is one, letting her know that this is to cease and desist at once, and if it does not, you will take further action against her on the grounds of insubordination. If you have any written evidence of this causing problems with clients, attach documentation to the note that the partners and HR see and place in her personnel file.