Stop Windows 10 Upgrade From Downloading
So you do not want to go to Windows 10 as you are happy with Windows 7 or 8 but the Windows 10 upgrade keeps nagging you or downloads when you don't have the data to cope with the download.
Here are links to explain how to stop the upgrade from downloading:
Disable Windows 10 Automatic Updates
You may be on an internet connection that only has a small amount of data but Windows updates are using all your data.
Even though I strongly believe you should leave Windows Updates to download you can prevent Windows 10 from doing this automatically.