First and most important part, obviously svMotion those VMs to other datastores in order to complete evacuate the source one. Move any other static files off as needed. For dismount procedure, see my article here.
Thanks for the inputs. Please confirm that the steps can be performed only from vCenter itself and not logging individual ESXi hosts. As the cluster contains more than 20 hosts its not easy to login with each host and perform operation of unmounting.
svmotion, unmounting datastore and then delete datastore activities from vCenter.
Thanks for clarifying
You should absolutely perform this from vCenter, not individual ESXi hosts.