Disable the Steam service. Run Steam only on a separate user session with limited rights (no admin and no access to your files). So essentially you'd have to manually switch user, via the login screen, to play your games.
As far as I'm aware, Steam makes its own folder writeable by everybody. Besides, you should get an error instead of the UAC if you're running as a non-admin account.
I moved to separate Wintendo box which is the best solution.