<?php

require_once 'Mage/Contacts/controllers/IndexController.php';

class Mynamespace_CustomContacts_IndexController extends Mage_Contacts_IndexController {

    protected function indexAction () {
        die;
    }
}

<config>
    <frontend>
        <routers>
            <contacts>
                <args>
                     <modules>
                        <Mynamespace_CustomContacts before="Mage_Contacts">Mynamespace_CustomContacts</Mynamespace_CustomContacts>
                    </modules>
                </args>
            </contacts>
        </routers>
    </frontend>
</config>

<?xml version="1.0"?>
<config>
    <modules>
        <Mynamespace_CustomContacts>
            <version>0.1.0</version>
        </Mynamespace_CustomContacts>
    </modules>
    <frontend>
        <routers>
            <contacts>
                <args>
                    <modules>
                        <Mynamespace_CustomContacts before="Mage_Contacts">Mynamespace_CustomContacts</Mynamespace_CustomContacts>
                    </modules>
                </args>
            </contacts>
        </routers>
    </frontend>
</config>

<modules>
        <Mynamespace_CustomContacts>
            <version>1.0.0</version>
        </Mynamespace_CustomContacts>
    </modules>

<Mynamespace_CustomContacts before="Mage_Contacts">
    Mynamespace_CustomContacts
</Mynamespace_CustomContacts>`

<mynamespace_customcontacts before="Mage_Contacts">
     Mynamespace_CustomContacts
</mynamespace_customcontacts>`