Turn off automatic checking for updates in the system options. That's what I did about a month ago and the banner popped up once saying an update was available. I dismissed it and no "reminders" since then.

You can try to block your Mi Box S's access to "android.googleapis.com" with your router. But that will also interfere with play store. Easier way is to stuff you box with apps and files and leave only ~1GB free space or less. Then it will say "Not enough free space to upgrade" when upgrade procedure starts.

I don't know, I've been using it since the update came out and have no problems. I only had to do a factory reset after the update to fix some minor glitches and it's been smooth sailing after that...