Recently, Apple switched to making the Sierra upgrade installer autodownload for users who have general autodownloads enabled.
If you want to discourage (but not prevent, especially if the users have admin privileges on their own machines) people from accidentally upgrading before you’ve had a chance to fully test Sierra (or any subsequent macOS releases), you can push (via Munki, for example) a fake installer.
For this, I just use Automator.
Save your “application” as Install macOS Sierra.app (or in subsequent years, whatever the new name of the macOS installer is).
Go ahead and deploy your fake installer by your usual means for managing your Mac fleet.