You simply need to send the letter informing the lawyer that you no longer want her to represent you. The letter should probably be sent certified mail/return receipt or priority mail with delivery confirmation.
If she is representing you in litigation, simply inform her to file a motion with the court informing them of the fact that she no longer represents you. If she refuses to inform the court, you should contact the clerk's office to file the appropriate paperwork. If this continues, you should contact the Georgia Bar.