To encourage more people to upgrade to macOS Sierra, Apple has enabled automatic downloading of macOS Sierra via the App Store app. Thankfully it won’t install the upgrade without your approval.
This auto download feature uses the existing software update feature of App Store app. It won’t download macOS Sierra if your Mac is low on storage or you have an older unsupported Mac.
You can disable automatic download if you have no intention to upgrade. Go to System Preferences > App Store, and uncheck “Automatically check for updates”.
Unfortunately this will also disable all updates including security updates. You will need to remember to do the updates manually.