There’s button in the App Store settings of System Preferences that lets you opt out of updates. The problem is that button doesn’t do anything if you’ve already checked for updates. This is very likely to happen since the first thing most people do after installing the new OS is open the App Store.
To turn off beta updates I first quit the App Store app and checked that there was a beta seed URL for Software Update using the Terminal:$ sudo defaults read \ /Library/Preferences/com.apple.SoftwareUpdate CatalogURL https://swscan.apple.com/content/catalogs/others/index-10.12seed-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz
I then nuked it:$ sudo defaults delete \ /Library/Preferences/com.apple.SoftwareUpdate CatalogURL
After getting off the beta, I then cleared the Software Update catalog with:$ sudo softwareupdate --clear-catalog Changed catalog to Apple production
As I did that the App Store settings in System Preferences immediately showed “No check for software updates has been done yet”. Launching the App Store refreshed the catalog and did not show the beta update.
mainly here to remind myself next time i do this