Sneaky, unwanted "upgrades" of people's computers to Windows 10 are -- amazingly -- getting even worse.
The latest is a dialog box that will schedule your computer for upgrading even if you click the red "X" box in the upper right.
There is a little application by Gibson Research (a very reputable outfit) called "never10" that will disable upgrading to Windows 10 in a safe manner and allow you to reverse the decision later if you so choose. It just sets all the settings to disallow the update and deletes the Windows 10 preparatory files that were sneakily downloaded probably without your even knowing it. If you want to change all the settings back to allowing Windows 10 upgrade, it will do that, too. It requires one button click to accomplish either choice -- so nice and easy.
Here is where you get it:
https://www.grc.com/never10.htmAt home and at my company, we have previously set things manually to preclude Windows 10, but with increased underhanded activity going on, I decided to run never10 on my home computer, and it found things that I missed doing it manually. Tomorrow, I am having never10 run on all computers in the company, including our mission-critical computers that run our production equipment. I might also send out a suggestion to customers to run it on computers that run the equipment they purchased from us.