I know this is a few months past but the only way I could disable automatic updates check was to use composer to capture it. You can click "Check for updates" at which point you check "Disable automatic checks for updates" and hit cancel. Once you check that and open it again to ensure you don't get prompted, you can finish your capture. I did this back in the summer last year so I'm not sure what files exactly were captured, but you could even fish through what you get in composer to find the specific plist file if you don't like pushing composed apps.