For what it's worth, the following terminal command seems to have cleared whatever bits were stuck:
sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL
I think the official way to turn off beta seeds via terminal is the command line tool "softwareupdate".
You can see a list of avalable updates on you current catalog via:
Reseting to the default one is done by:
sudo softwareupdate --clear-catalog
I had the same problem and it solved it for me.