Sorry, no. The option to prevent Steam from updating games is not 100% reliable and is not limited to Skyrim. To complain about this, I would suggest going to the Steam forums, as this is not something Bethesda has control over.
To take precautions against this, you should make a backup of your game folder whenever a new beta becomes available. When that beta goes live, there is a good chance your game will be updated without your consent. And don't ask for copies of game files from previous versions, as that will only provoke the zealotry of the moderators and get your thread locked.