1st, password protect the admin. account. Then, from the admin log-on, right click on the STEAM program icon, click properties, then, on the compatibility tab. Near the bottom of that window, check the box that says "Run This Program as an Administrator"
From now on, standard users cannot access the program without a password.
Or, if you install the program to C:\users\username (name of admin account) instead of C:\program files, standard users won't even be able to see the program installed.