Prevent wp-login.php from being used

We currently use a different authentication mechanism for our blogs service (Shibboleth) and don’t want anyone to be able to log in via wp-login.php, other than the main administrative user. Anyone aware of a plugin that’ll do that for me?

So even if a username is valid, if it isn’t the administrator account, there is no successful login at wp-login.php.

Suggestions?

Thank you.