How to stop Mac App Store from downloading macOS Sierra?
Even though you already installed macOS Sierra, App Store still allows you to download the original install file.

There are 3 methods to cancel a download in Mac App Store:


You can stop the download by going to Launchpad, click and hold on macOS Sierra and click the small 'x' button to delete it. This will stop its download.
Turn off your WiFi temporarily and Mac App Store will show a message that the download has failed. Then, find the downloaded file in the Applications folder and move it to Trash. Empty the trash and turn your WiFi back on.
Go to the Purchases page in Mac App Store, hold down option key and click Cancel. The download will be cancelled.