How do I disable the nagging to update to High Sierra?
There are two ways to do this. The first should just prevent the prompt to update to the newest OS while the second will disable the App Store from bothering you altogether to update anything.

The most recommended is to go to the App Store, select the Updates tab, right click on the update you wish to stop being bothered about, and select "Hide Update".

The alternative is to go to System Preferences, click on App Store, deselect "Automatically check for updates". This should prevent the App Store from bothering you about all available update.