From documentation:
Remove VMFS Datastores in the vSphere Web Client
Note
The datastore delete operation permanently deletes all files associated with virtual machines on the datastore. Although you can delete the datastore without unmounting, it is preferable that you unmount the datastore first.
To safety unmount the datastore, follow the procedures from KB: VMware KB: Unmounting a LUN or detaching a datastore/storage device from multiple VMware ESXi 5.x hosts