There is a need to remove a scanner from the list of appliances within the user interface of Advanced Threat Protection
In ATP versions 2.0 - 2.2 it is not possible to remove obsolete scanners directly from the GUI, once scanner is powered down it will change the state to "Disabled" and continue to appear in the scanner list.
The ability to remove a scanner from the ATP Manager web interface is available in ATP 2.3 and later. To delete a network scanner, follow these steps:
Wait a few minutes to ensure that the appliance is completely shut down.
Note: The Delete Appliance icon only appears for appliances that have the role of Scanner and are also upgraded to ATP 2.3. Scanners still on ATP 2.2 cannot be deleted this way.
For further details, please review page 55 on the Advanced Threat Protection 2.3 Administration Guide here: https://support.symantec.com/en_US/article.DOC9664.html