Hi,
You may add the following point in the Appointment letter.
" The Company will be entitled to terminate your employment by giving you, 1 Month written notice. In case you want to terminate this employment , you will be required to give 1 Month written notice or forfeit in lieu thereof, the gross pay for 1 Month or pro-rated for the period by which such notice falls short of 1 Month.".
Also during resignation discussion with the employee make it clear to the employee that one month notice is compulsory and proper relieving will be given only if the employee serves proper notice period.