Sorry guys, you cant disable autoupdate on steam (don't believe the properties menu, it lies). Any attempts to install or launch the game will automatically override the do not update option. And of course steam will not allow you to play the game in offline mode, so that's a no-go as well. Just be patient and hope that firaxis doesn't go update crazy like stardock. That or pray someone discovers a magical .ini file that disables the override script.
Either that or wait from some enterprising collective to remove steam from the equation.
[Edited by Evocati, 9/23/2010 7:56:06 PM]
Auto-update != Mandatory patches.
Besides, you can just go into offline mode if you want to keep the version, though that requires you do it beforehand.