Bingo, you can no longer opt out of any updates on Steam. Valve even changed the wording of the client option (for ALL games, not just this one) to say "Only update this game when I launch it", rather than something like "Do not keep this game up to date."
They used to have a dialog box that let you opt out of updates:
You could choose "Launch Now" if an update was queued but not yet downloaded. Now you can't even do that anymore. I assume the only option is manually backing up game files and trying to restore them after the Steam update finishes.