Here, you can find some tips/notes to manage Apple Software Updates from the Terminal/Script.
Disable Apple Software Update automatic check:
/usr/sbin/softwareupdate --schedule off
Disable the download of new update in the background:
/usr/bin/defaults write /Library/Preferences/com.apple.SoftwareUpdate AutomaticDownload -bool false
Disable system data files and security updates:
/usr/bin/defaults write /Library/Preferences/com.apple.SoftwareUpdate ConfigDataInstall -bool falses
Disable the auto-install of the critical updates:
/usr/bin/defaults write /Library/Preferences/com.apple.SoftwareUpdate CriticalUpdateInstall -bool false