To properly disable the Windows 10 upgrade as recommended by microsoft I would advise reading the following article:
Ref: https://support.microsoft.com/en-us/kb/3080351
Computer Configuration
To block the upgrade by using Computer Configuration, follow these steps
Click Computer Configuration.
Click Policies.
Click Administrative Templates.
Click Windows Components.
Click Windows Update.
Double-click Turn off the upgrade to the latest version of Windows through Windows Update.
Click Enable.
Policy path
Computer Configuration / Administrative Templates / Windows Components / Windows Update Policy
Setting: Turn off the upgrade to the latest version of Windows through Windows Update
Windows registry
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:
Subkey: HKLM\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate
DWORD value: DisableOSUpgrade = 1