Hi,
I think you should check those 2 links about lockfiles :
VMware Knowledge Base (Manipulating a file on a Datastore fails with the error: device or resource busy / file is locked (1008728)
VMware Knowledge Base (Investigating virtual machine file locks on ESXi (10051))
Else, have you checked if the volume is in use ? If you can , temporarly remove any links between your VMs and the datastore then unmap -l the datastore then rebuild links.
If you use HA check if your datastore is not used for heartbeat (let me know if you need help)