Dear Manaswita,
I can understand your problem, as I am also facing and have faced this earlier. Managing people is very difficult and complicated, even though they might have good skills and were good workers before. Attitude and behavior will change due to professional and personal reasons. Just find out what is wrong with her, why she is acting like that, which was not the case before.
After doing the counseling, motivating, warning, issuing notices, giving a lower grade in the appraisals, escalating the matter to higher-ups, and nothing works, just issue a final warning notice and go as per the terms of the appointment order to terminate her. Build a case file, record all the facts that you considered, and measures that you took to retain her; this will help you in justifying your actions.
All the very best.
Regards,
Ramachandra