You might consider only going online with a bootable Linux disc, as then nothing can touch your system and then do whatever other work you need to do on your regular ‘doze install after you reboot and never allow your machine to access the internet unless you are running Linux.
That way, you can still browse the web and access your email among other things, but when you reboot and disable your modem/gateway/networkaccess link you will not be troubled with the verdammt “automatic-upgrade” problem again.
Just a thought.
Does your suggestion imply a read-only disk drive and a read-only disk?