Despicable.
[Ctrl]-[Alt]-[Del]
OR...
Pull the power cord, then unplug the network line (ethernet, dsl, etc.)...
Then reboot into Safe Mode and allow the computer to recover and check all drives. Then reboot again (without connecting the network cable) and uncheck the “allow win10 upgrade” popup box.
Shutdown, connect cables, power up and search for something called Never10 (I think. Someone will chime in in a bit and point it out).
Oh, and of course look for a- ... -an alternative Operating System such as Linux as soon as you can realize what others already have: u-squish (MS) is still actively looking for a way to force you to “upgrade” one way or another. It is only a matter of time until you are assimiliated.
Go Linux.
Or Apple, if you absolutely have to. To each his own.
Easy fix: don’t use an administrative user. Windows 10 requires admin rights to upgrade. If you use a non-admin account on your system, you won’t be pestered.
This is how OSX and Linux work. People like Windows for its convenience without realizing that they run as an admin natively. Try changing that. It doesn’t take much effort.