Sometimes you just need to tell people something before they log in. Like, if your name is Pat Griffin, you’re not allowed. (Pat Griffin is a friend of mine I worked with. We like to give each other a hard time. This is the kind of thing he would love.)

/*
    Add custom message to WordPress login page
*/
function SVS_login_message( $message ) {
    if ( empty($message) ){
        return "<p><strong>This login is only for Site Admins. If you are something else, please click the logo above and use the LOGIN link in the main menu.</strong></p>";
    } else {
        return $message;
    }
}
add_filter( 'login_message', 'SVS_login_message' );