unwanted application - How can I call off a Windows 10 upgrade?

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)?

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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…