Hi,
I have faced this challenge a couple of times and agree that even if what to safeguard the Company's interest, we cannot Force an employee to work out the entire period. We usually persuade the employee to complete the handover process and release them before the notice period. If you force them to work, they turn the workplace TOXIC. That will harm you more then 15-20 days of their notice period. As soon as an employee resigns, the handover process comes into play. You never know when an happy ex-employee rejoins or helps your company by spreading the 'Good Word" about their last organization.
Thanks,
Alex