Have you tried “GWX Stopper”...seemed to work on my mom’s Win 7 Pro machine.
http://blog.ultimateoutsider.com/2015/08/using-gwx-stopper-to-permanently-remove.html
(I am OS X as of 3 years ago, not looking back).
Right, then -ah, I am not familiar with “GWX Stopper” but if it is more than a couple of months old I am guessing that it is a simple shell script to ‘manually’ edit the registry files to delete the You-Want-To-Install-Win10 section, however it has been previously noted that in the last MS ‘upgrade’ or several, ‘doze now automatically (without your consent) checks to see if you have “upgraded” or not and resets the nag screen if you have not to ensure you are “safe”
-and “upgraded”.
Just FYI.