so, while there is no way to uninstall just the 2.1 patch that i know of, there IS a way to stop Steam from applying it in the first place. go to your library of games and right click on the game in question, in this case Witcher 2. a context menu will pop-up with various options, the last one of which is properties. click on properties, and a new window appears.the 2nd tab of this window is titled updates. this is where you can tell Steam to not automaticslly update any game. i suppose it might be possible to fully uninstall the Witcher 2 game, and then reinstall it, and before 2.1 is added, set it to not auto update, but as i do not have the steam version i will leave that to you to find out.
hope this helps someone out there