To prevent Steam from auto-updating, you must perform the following steps:

    Create a text file that contains the following line: BootStrapperInhibitAll=Enable
    Save this file with the name Steam.cfg.
    Locate the Steam application, then right-click and select "Show Package Contents".
    Open the folder "Contents", then "MacOS".
    Drag or copy the "Steam.cfg" file from Step 1 to this location.
    Close any extra Finder windows, then double-click "Steam" to run as normal.