Check that any iso files (from this datastore) is not attached in a VM.
the data store is empty.
Please try unmounting it from CLI
1. List the datastore name
# esxcfg-nas -l
2. Delete the datastore
# esxcfg-nas -d <NFS_Datastore_Name>
or
# esxcli storage nfs remove -v vol_nfs-2