New Group Policy option to disable upgrades
Perhaps it might have been useful to mention that an update released in December, Microsoft KB3112343, also updates the C:\Windows\PolicyDefinitions\WindowsUpdate.admx file and adds an option "DisableOSUpgrade" with the notes saying:
If you enable this setting, Windows Update will not offer you an upgrade to the latest version of Windows.
If you disable or do not configure this setting, Windows Update might offer an upgrade to the latest version of Windows
This update is for Win 7, but the group policy setting also applies to Win 8.1 Update.