This code will remove the MarketPress registration PopUp after order. Simply copy and past in functions.php file or use as mu-plugin.

mp_after_order_remove_register_lightbox.php

<?php

if ( ! function_exists( 'remove_anonymous_object_filter' ) )
{
    /**
     * Remove an anonymous object filter.
     *
     * @param  string $tag    Hook name.
     * @param  string $class  Class name
     * @param  string $method Method name
     * @return void
     */
    function remove_anonymous_object_filter( $tag, $class, $method )
    {
        $filters = $GLOBALS['wp_filter'][ $tag ];

        if ( empty ( $filters ) )
        {
            return;
        }

        foreach ( $filters as $priority => $filter )
        {
            foreach ( $filter as $identifier => $function )
            {
                if ( is_array( $function)
                    and is_a( $function['function'][0], $class )
                    and $method === $function['function'][1]
                )
                {
                    remove_filter(
                        $tag,
                        array ( $function['function'][0], $method ),
                        $priority
                    );
                }
            }
        }
    }
}


add_action( 'wp_footer', 'kill_anonymous_filter', 0 );

function kill_anonymous_filter()
{
    remove_anonymous_object_filter(
        'wp_footer',
        'MP_Public',
        'create_account_lightbox_html'
    );
}