How to disable Multisite sign-up page?

[Update]

An alternative (maybe better) is to use the following constant in wp-config.php:

define( 'NOBLOGREDIRECT', 'http://example.com' );

---

At the very beginning of wp-signup.php file there is this code:

function do_signup_header() {
    do_action( 'signup_header' );
}
add_action( 'wp_head', 'do_signup_header' );

So, it’s just a matter of adding the signup_header action hook to break any further execution and redirect the browser to other URL.

Here, wrapped as a Must Use Plugin:

<?php
/*
    Plugin Name: Multisite - Prevent Sign-up Page
    Plugin Url: http://wordpress.stackexchange.com/q/85529/12615
    Version: 1.0
    Author: Rodolfo Buaiz
*/

add_action( 'signup_header', 'rbz_prevent_multisite_signup' );

function rbz_prevent_multisite_signup() 
{
    wp_redirect( site_url() );
    die();
}