Example of how you can override the PROMO.update_payment_info_table method. This is a copy of the original function with the code to automatically submit the registration when the payment amount == 0 commented out.

example.php

<?php //Please do not include the opening PHP tag if you already have one

function tw_my_custom_promo_submit(){
    wp_add_inline_script( 
        'espresso_promotions',
        "jQuery( document ).ready(function() {
            PROMO.update_payment_info_table = function( response ) {
                //SPCO.console_log( 'payment_info', response.return_data.payment_info, true );
                jQuery('#spco-payment-info-table' ).find('tbody').html( response.return_data.payment_info );
                SPCO.scroll_to_top_and_display_messages( SPCO.main_container, response, true );
                if ( typeof response.return_data.cart_total !== 'undefined' ) {
                    var payment_amount = parseFloat( response.return_data.cart_total );
                    SPCO.main_container.trigger( 'spco_payment_amount', [ payment_amount ] );
                    if ( payment_amount === 0 ) {
                        SPCO.enable_submit_buttons();
                        //SPCO.main_container.find( '.spco-next-step-btn' ).trigger( 'click' );
                    }
                }
            }
        });"
    );
}
add_action( 'wp_enqueue_scripts', 'tw_my_custom_promo_submit', 50 );