While attempting to delete VM Templates from either vCenter Server's Content Library Section or from the vCenter Server Inventory, the delete operation fails with the message:
Task Name: Delete Library Item
Targe: VM Name
Status: The operation is not supported on the object.
vCenter Server
The objects have already been deleted from the backing datastore, and therefore there is nothing to delete from the vCenter Server's perspective.
Delete the VM Templates from vCenter inventory using "Remove from inventory" option.