Specifically, remove the `printAdminToolbox()` function call from your theme. Then enable the `user_login-out` plugin and add a call to `printUserLogin_out()` where you want the logout link.
But as acrylian says, this is no protection. If you allow users to share a usercode they can mess with eachother. Better to use the gallery "guest" logon for this type of access. You can set that up with a user id and password so that these users will not be able to tell the difference other than that they cannot access the back-end.