Dear Sheeba,
Have you mentioned the terms and conditions of separation in the appointment letter? If yes, then what are those? If she has not fulfilled those conditions, then you may send her an official letter stating that her resignation is not accepted. You may write that "as per the provisions of paragraph No ______ of the appointment letter dated _____ issued to you, you are required to complete a notice period of ____ days. If you do not report for your duties, then it will be considered that you have abandoned your duties and appropriate disciplinary action will be initiated against you".
Send her the hard copy of the letter by Speed Post. Keep the duplicate copy with you. Keep the receipt from the post office also with you. Scan the duplicate copy along with the post office receipt, send it by email to her.
If the employee does not report for her duties, then send one more official letter. This time, you may mark a copy of the letter to the MD of her company.
Let us see what happens when you complete all the above actions.
By the way, while seeking advice from the seniors, it is pertinent to give the background of the case. It speaks poorly of HR or any other professional to write curt posts and seek advice.
Thanks,
Dinesh Divekar