Limit automatic updates to software and applications only, but not OSX
It will not automatically update to Sierra.  It will alert you, and will download in the background (one of the "features" they copied from Windows 10) but you have to click on the "Install" button to install it.  

I am running El Capitan with everything set to Auto Update and not once has it tried to install Sierra.  However, the way I got around it was that I downloaded it to make a USB Installer and left it in place.

That said, you can turn off the background downloading of automatic updates.  In System Preferences --> App Store:



You also have the option to install app updates and OS X updates, but it will download items in the background.