There’s a few odd things about High Sierra and you will be getting constant notifications telling you to upgrade. You can’t even dismiss the notification: the options available to you are ‘Install’ which is the one thing you don’t want to do. Or ‘Details’ so you can find out more about the one thing you don’t want to do.
But bravely right click on the banner and there is a prompt “Hide Update” option. To cease unnecessary notifications, the simple right click, open the Mac App Store and bravely right-click the banner to reveal the “Hide Update” option, once selected those pesky notifications should stop.