29th November 2018 From India, Bangalore
Their is nothing as such a Term mentioned in the law about the notice period. The states that is any employee resigns he/she should give proper handing over and obtain a clearance, however there is nothing that Notice period of so and so months have to give.
The practice that you have adopted is of Forced Labour which at form my replicate to you policy.
Better to change your policy and implement as per legal Laws.
30th November 2018