How can I call off a Windows 10 upgrade? [duplicate]

I have a machine with Windows 7 that was set to fully automatic Windows Updates (trusting fool that I was). Tonight I was unpleasantly surprised by a window that came up seemingly out of nowhere, telling me Windows 10 was ready to install, which looked like this:

The only options, as you can see, were "schedule it for later" and "start the upgrade now" — not even a close button in the corner. I considered killing the process behind it, but checking Windows Update revealed that it was merely waiting for me to reboot to plow forward, come hell or high water. I chose "schedule it for later", thinking I'd just set it for sometime next century, but the only date choices were today, tomorrow, and the next day. Three choices exactly. So I picked the last possible date and time.

So. Now the race is on. How do I abort this unwelcome mess they're trying to shove down my throat before they do it anyway?

Bonus question: people around the web point out that this thing downloaded gigs of stuff silently, in the background, without asking, in preparation for this attack. Can I recover that disk space as well (if not my data usage nor my trust)?

Update:

Later that night, I found the C:\$windows.~BT folder as directed, and wiped it out (having to struggle with permissions in order to do so, I might add). I wondered if that might be the end of it. Lo and behold, out of nowhere:

The process of this window is C:\Windows\System32\wuauclt.exe — Windows Update. The folder I wiped out is still gone, though. And as before, I'm only allowed to put it off by two more days:

It sprung up, counting down from one hour. If I leave my machine running and miss that one hour window, I'm screwed. Apparently I just have to keep averting disaster every couple of days for the rest of eternity. Thanks, Microsoft.

Can anyone tell me how to kill this zombie permanently??

Update 2:

It looks like Windows Update is no longer capable of receiving updates, since the Windows 10 upgrade is pending:

Update 3:

I made the registry changes suggested in the other question as well. The appointed time came Sunday, and the machine indeed rebooted itself unasked, and there's something in the log about a failed upgrade-to-Windows-10 attempt, but I seem to have averted disaster. For now…

Windows 10 gets downloaded to the following (I think hidden) folder:

C:\$windows.~BT

You can try to delete that folder, or use the built-in windows function to free the disk space (right click on the hard drive->properties->tools).

If you have deleted it successfully then you can disable the windows update

Edit: This may be worth try:

Stop Windows 10 Upgrade

In a few words: Go to your Windows Update history and delete Update "KB3035583". After deleting it you may want to hide it so it does not get downloaded again. Maybe this can help you to stop the Microsoft Upgrade process.

If this still fails you may need to delete the "GWX.exe" file in /Windows/SysWOW64. To do that you have to take ownership of that folder.

Read more about it here:

Stop Upgrade Part2