Dear Dmitriy,
Sorry had no time to develop plugin, but i have modified: logincontroller.php to accept only internal IP address, here is file:
<?php
/*
 * This file is a part of Mibew Messenger.
 *
 * Copyright 2005-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     
http://www.apache.org/licenses/LICENSE-2.0 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
namespace Mibew\Controller;
use Symfony\Component\HttpFoundation\Request;
/**
 * Contains acctions related with operator login process.
 */
function get_IP_address()
{
    foreach (array('HTTP_CLIENT_IP',
                   'HTTP_X_FORWARDED_FOR',
                   'HTTP_X_FORWARDED',
                   'HTTP_X_CLUSTER_CLIENT_IP',
                   'HTTP_FORWARDED_FOR',
                   'HTTP_FORWARDED',
                   'REMOTE_ADDR') as $key){
        if (array_key_exists($key, $_SERVER) === true){
            foreach (explode(',', $_SERVER[$key]) as $IPaddress){
                $IPaddress = trim($IPaddress); // Just to be safe
                if (filter_var($IPaddress,
                               FILTER_VALIDATE_IP,
                               FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE)
                    !== false) {
                    return $IPaddress;
                }
            }
        }
    }
}
$PublicIP = get_IP_address();
if ($PublicIP== "") {
class LoginController extends AbstractController
{
    /**
     * Builds a page with login form.
     *
     * @param Request $request Incoming request.
     * @return string Rendered page content.
     */
    public function showFormAction(Request $request)
    {
        // Check if the operator already logged in
        if ($this->getOperator()) {
            // Redirect the operator to home page.
            // TODO: Use a route for URI generation.
            return $this->redirect($request->getUriForPath('/operator'));
        }
        $page = array(
            'formisRemember' => true,
            'version' => MIBEW_VERSION,
            // Use errors list stored in the request. We need to do so to have
            // an ability to pass the request from the "submitForm" action.
            'errors' => $request->attributes->get('errors', array()),
        );
        // Try to get login from the request.
        if ($request->request->has('login')) {
            $page['formlogin'] = $request->request->get('login');
        } elseif ($request->query->has('login')) {
            $login = $request->query->get('login');
            if (preg_match("/^(\w{1,15})$/", $login)) {
                $page['formlogin'] = $login;
            }
        }
        $page['localeLinks'] = get_locale_links();
        $page['title'] = getlocal('Login');
        $page['headertitle'] = getlocal('Mibew Messenger');
        $page['show_small_login'] = false;
        $page['fixedwrap'] = true;
        return $this->render('login', $page);
    }
    /**
     * Processes submitting of the form which is generated in
     * {@link \Mibew\Controller\LoginController::showFormAction()} method.
     *
     * Triggers 'operatorLogin' event after operator logged in and pass to it an
     * associative array with following items:
     *  - 'operator': array of the logged in operator info;
     *  - 'remember': boolean, indicates if system should remember operator.
     *
     * @param Request $request Incoming request.
     * @return string Rendered page content.
     */
    public function submitFormAction(Request $request)
    {
        csrf_check_token($request);
        $login = $request->request->get('login');
        $password = $request->request->get('password');
        $remember = $request->request->get('isRemember') == 'on';
        $errors = array();
        $operator = operator_by_login($login);
        $operator_can_login = $operator
            && isset($operator['vcpassword'])
            && check_password_hash($operator['vclogin'], $password, $operator['vcpassword'])
            && !operator_is_disabled($operator);
        if ($operator_can_login) {
            // Login the operator to the system
            $this->getAuthenticationManager()->loginOperator($operator, $remember);
            // Redirect the current operator to the needed page.
            $target = isset($_SESSION[SESSION_PREFIX . 'backpath'])
                ? $_SESSION[SESSION_PREFIX . 'backpath']
                : $request->getUriForPath('/operator');
            return $this->redirect($target);
        } else {
            if (operator_is_disabled($operator)) {
                $errors[] = getlocal('Your account is temporarily blocked. Please contact system administrator.');
            } else {
                $errors[] = getlocal("Entered login/password is incorrect");
            }
        }
        // Rebuild login form
        $request->attributes->set('errors', $errors);
        return $this->showFormAction($request);
    }
}
}