I doubt it would work in my case as my user is already the owner of that folder. Perhaps denying write permissions for all users for each game would do the trick? For instance, you could deny "write" for all users (Administrators, Users, Authenticated Users and System) for let's say C:\Steam\SteamApps\common\Fallout New Vegas, and that would basically make the folder and everything in it "read-only". I don't know what would happen if Steam tried to change something in it though, would probably throw an error.
By the way, I've been using the steam.cfg trick to prevent constant client updates. Don't know if it still works as I haven't really launched Steam for quite a while now. You can google it if you wish, but you simply place a steam.cfg text file into Steam folder with a single line, which is BootStrapperInhibitAll=Enable.