If you've already got macOS Sierra installed on your Mac, or you don't have a problem with Apple beaming it onto your hard drive, then you can stop reading now.
However, if you'd rather not have an entire operating system downloaded to your Mac in the background -- with the resultant effect that might have on your Wi-Fi connection -- or your Mac is downloading it despite your hardware not being compatible (there are third-party reports of this happening) then read on.
Fortunately, it's easy to block the update.
From the menu bar click on the Apple menu (the Apple icon in the top-left) and choose System Preferences
From there select App Store
Under Automatically check for updates, uncheck Download newly available updates in the background
It's that simple.
Macs that are compatible with macOS Sierra are as follows: