I stopped unexpected Windows 10 update. How to block Windows 10 update permanently?

While checking out the user's Windows 7 Pro laptop, I checked off the Definition Update for Microsoft Security Essentials.  When I started the update after the selection, I saw a message on the Windows Update that Windows 10 was being downloaded.

This person doesn't need Windows 10.  

I don't know why this appeared.  How can I block it?
For now, I've changed the setting to Download but let me choose whether to install them.

I would prefer automatic installation of important updates, but I don't want Windows 10 to auto-install.

Thanks.