It's necessary to configure the client purge setting in a Symantec Endpoint Protection Manager (SEPM) running version 12.1 RU1 (12.1.1000.157).
This setting controls the number of days before a non-communicating Symantec Endpoint Protection (SEP) client is removed from the SEPM's database.
The location of this setting is different in SEPM 12.1 RU1.
To access the client purge configuration in a 12.1 RU1 SEPM go to the Admin page of the console, select Domains and then edit the appropriate domain properties.
On the General tab is a checkbox for 'Delete clients that have not connected for specified time', with a field to input the number of days next to this option.
From here it is possible to control client purge options in a 12.1 RU1 SEPM.