Resolution 1
In the example below, the highlighted domain controller was decommissioned without uninstalling the audit agent first.

How to remove the decommissioned domain controller from the audit agents list:
From the "More" drop-down menu, select "Excluded Domain Controllers".

Add the decommissioned server to the "Excluded" list:

Refresh the audit agents page, and the excluded decommissioned domain controller will no longer appear:

Resolution 2
This option allows you to remove the decommissioned domain controller via SQL.
IMPORTANT NOTE: Verify that there is a current backup copy of the database before making any changes.

The decommissioned DC should no longer appear in the Agents list.