Basically the only way to stop the automatic countdown restart after an important update is done is to Lock the screen.
This is a quote from Microsoft:
"After you install an important update in Windows 8 or in Windows Server 2012, you receive a notice that you have to restart the computer in three days. If the restart does not occur in three days, the computer displays a 15-minute countdown and then automatically restarts. By default, this automatic restart is delayed if the computer is locked, and the countdown will begin the next time that you sign in to the computer"
The following fix allows you to force the server to restart after the update have been applied if it requires a restart.
If you want to Allow configuration of Automatic Updates in Windows 8 and Windows Server 2012 then please read this: