How can I stop my Mac from automatically upgrading to High Sierra? As per the title. If I have a Mac running Sierra, what can/should I do if I don’t want to be upgraded to High Sierra?

I’ve already unchecked “Install macOS updates” in System Preferences:



but I’m wondering if there’s anything else I should be doing. (Or can do, because I’m not sure I trust that checkbox.)



Why?

This weekend, one of my Macs got unexpectedly upgraded from Sierra to High Sierra, but I don’t remember asking for the upgrade. (I say “upgraded”, what I mean is “freeze, crash, then reboot twice”. Is this a new installation procedure?) A bunch of stuff is now broken, yay.

I’m downgrading to a backup taken when my Mac was on Sierra. When I’m back up, I want to ensure I don’t get another unwanted upgrade.

Normally I would just follow a policy of “don’t hit the upgrade button”, but this weekend’s experience suggests that’s no longer enough. :-/
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.