In one particural case, you don’t want to disable WP registration but just redirect the page so some other script takes care of it. That happened to me while working on a WordPress and bbPress website. The solution is quite simple:

add_action(‘login_form_register’, ‘redirectregister’);
function redirectregister(){wp_redirect(get_option(‘siteurl’) . ‘/bb/register.php’);}

add this where wordpress will find it, in your functions file or just any plugin.