As skrebbel said, don't they charge for the upgrade now? That said, Never10[1] was (still is?) a great tool to prevent the Windows 10 auto-upgrade. Also, according to the Never10 page, Microsoft now has an optional update to get rid of the GWX stuff.[2]

[1] https://www.grc.com/never10.htm

[2] https://support.microsoft.com/en-us/kb/3184143