See https://twitter.com/taupecat/status/562625344289927168

only_certain_logins_allowed.php

<?php
/**
 * Plugin Name: Only Certain Logins Allowed
 * Description: Only let certain usernames do anything...
 * Version: 1.0
 * Author: Russell Heimlich
 * Author URI: https://twitter.com/kingkool68
 */
 
function only_certain_logins_allowed() {
	$allowed_usernames = array( 'trotton', 'rheimlich' );
	$user = wp_get_current_user();
	if( !in_array( $user->user_login, $allowed_usernames) ) {
		wp_die( 'Maintence Mode: you\'re not allowed. Ask Tracy ¯\_(ツ)_/¯' );
	}
}
add_action( 'init', 'only_certain_logins_allowed' );