Microsoft released “official” instructions on how to stop Windows 7 & 8 from upgrading to Win10:
To block the upgrade to Windows 10 through Windows Update, specify the following registry value:
Subkey: HKLM\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate
DWORD value: DisableOSUpgrade = 1
For non-Enterprise versions of Windows, the (GWX) notification icon can be suppressed through the Windows registry. To do this, set the following registry value:
Subkey: HKLM\Software\Policies\Microsoft\Windows\Gwx
DWORD value: DisableGwx = 1