When Datastores are still on VMFS-5, reclaiming disk space is a manual process. VMware KB “Using the esxcli storage vmfs unmap command to reclaim VMFS deleted blocks on thin-provisioned LUNs (2057513)” details how to use the unmap command.
The action is performed on an ESXi hosts, the basic command looks like this:
# esxcli storage vmfs unmap -l <Volume label>
Where Volume label is the human readable name of a Datastore like: “VMFS01”.
Depending on the size of the datastore(s), running unmap will take quite some time. If you have few datastores, you run this command a couple of times and voila. If cluster(s) have dozens of datastores, the following workaround can help you.