ONUs can be managed from the corresponding view in XOS, it is located under vOLT -> ONU Devices
From this view you can view all the ONUs known to the system, remove them or go to the details page (clicking on the magnifier icon)
Once you are in the ONU details view, you can just change it's Admin State
to ENABLED
if you wish the device to be enabled, or ADMIN_DISABLED
if you wish the device to be administratively disabled. Avoid manunally setting the state to DISABLED
as the workflow may toggle it back to ENABLED
.