Wait wait wait...
How administrator enter to site? Without login and password it's impossible.
If admin enter to site with login and password, then in logout function clear session vars.
For example:
$_SESSION["pass"] = '';
Yes, other user (on the same PC) can click on the "Admin link", but now they wil be see "Incorrect password, Please enter corrcet password". =)
...or i am inderstand your question.... =)