Unlike the unmount operations, the delete operation will destroy all the data on the datastore. Once done, you cannot revert this operation. Hence, make sure to move all the virtual machine data currently on the datastore to another datastore.
The following procedure will guide you to delete a datastore from the vSphere Web Client:
Use the vSphere Web Client to connect to the vCenter Server.
Navigate to Home | Storage.
With the Datacenter object selected, navigate to Related Objects | Datastores operation and identify the VMFS datastore to be deleted:
Right-click on the datastore to be deleted and navigate to All vCenter Actions | Delete Datastore:
Verify the Recent Tasks pane, which should show a Remove datastore completed successfully: