Setting the "No auto-restart with logged on users for scheduled automatic updates installations" GPO didn't work for me. I woke up this morning and my machine had been rebooted. (I was using the computer last night and didn't even get a warning.)
This suggestion from techjourney.net looks more promising.
Windows will attempt to re-enable the task automatically, so you also need to modify permissions on the task file.
Allez dans C:\Windows\System32\Tasks\Microsoft\Windows\UpdateOrchestrator
Ouvrez le properties of the Reboot file, ouvrez le Security tab and open Advanced Permissions.
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Take ownership of the file by clicking the "Change" link next to the owner and typing in your name. Disable inheritance via the button en bas.
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Change all permissions to read or read & execute only.