Block any kind of logins to the site

block-login.php

<?php
/**
 * Plugin Name: Block Login
 */

function no_login_authenticate_filter( $user, $username, $pass ) {
	remove_filter( 'authenticate', 'wp_authenticate_username_password', 20, 3 );

	return null;
}

add_filter( 'authenticate', 'no_login_authenticate_filter', 1, 3 );