To properly disable the Windows 10 upgrade as recommended by microsoft I would advise reading the following article:
To block the upgrade by using Computer Configuration, follow these steps
Click Computer Configuration.
Click Administrative Templates.
Click Windows Components.
Click Windows Update.
Double-click Turn off the upgrade to the latest version of Windows through Windows Update.
Computer Configuration / Administrative Templates / Windows Components / Windows Update Policy
Setting: Turn off the upgrade to the latest version of Windows through Windows Update
Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
To suppress this offer through the registry, specify the following registry value:
DWORD value: DisableOSUpgrade = 1