How can I stop my Mac from automatically upgrading to High Sierra?
I'm not aware that anything has changed in terms of macOS upgrades. 

The only Mac I have with macOS High Sierra actually shipped with it, while all the others regularly inform me of the upgrade, but none have actually been upgraded to macOS High Sierra. The ones that support macOS High Sierra but haven't upgraded are either running El Capitan 10.11.6 or Sierra 10.12.6.

So, based on your settings, you should be fine.