If the task is still in progress then try to kill it at command line:
VMware KB: Powering off an unresponsive virtual machine on an ...
kb.vmware.com/kb/1004340
This should at least do something with the task in vCenter.
If you cannot locate a process/vm world, then you may need to restart vCenter.
You can also use the vsphere client to connect direct to the ESXi host, and see if the tasks are in progress there too (to see if there is a disconnect between vCenter and ESXi).
at the end of it all, then you may need to restart the Lab Manager monitor service to clear the task there too.