There are far simpler and less risky ways to achieve this.
I don't have this issue with my Etrex 30. I see you don't own an Edge series cyclometer. Express automatically updates the Edge firmware and other than tricking the update version check there is no way to use Express and avoid auto updates. The one exception is to navigate to the Garmin folder and delete gupdate.exe every time you connect. That's not very practical. Or Express can be disabled and activities can be manually uploaded.
A description of simpler ways to accomplish avoiding auto firmware updates on the Edge series devices would surly be appreciated.
You've sort of answered it yourself without any need to use the RGN tool that many may not be comfortable with. The gupdate.gcd file only works when you boot your GPS so if you delete it then no update.
