Hi,
When an employee fails to serve proper notice period as per the terms and conditions of his /her appointment order he /she is not eligible for Relieving /Experience certificate.
If the employee absconds you need to follow disciplinary process : Warning /termination letters.
If the employee didn't absconded but insisting for immediate relief, then take his request in writing and then you may issue a termination letter stating vide your resignation letter dated __you had submitted resignation but had expressed your inability to serve proper notice period.. hence you are services are being terminated by the closing hours of____