Originally posted by TheBDK:
Offline mode is your best bet, sadly you can only use it for 2 weeks before drm mode kicks in to annoy you. Typical steam.

Tell that to the people who have been using the offline mode for longer than that.
And a Valve Employee negating that restriction

Anyway. If you want to disable automatic updating, Trying the following is worth a shot:

Open the Windows run dialog (Windows key + R) and type in steam://open/console to open the Steam console.

Then type in:

@AllowSkipGameUpdate 1

Hey presto, Steam will now let you skip updates when you launch a game.