you need to edit the actual code itself, the server behaviors trigger does allow you to set trigger that will alwas be false, you need to do that manually.
on the confirm page, you will need to find the code for the authorize.net server behavior, the first few lines will look like:
<?php
if (($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST)) {
$AuthNet_required = array();
$AuthNet_required[0] = array();
$AuthNet_required[1] = array();
you will need to edit this to:
<?php
$<your cart object name>->redirStr = "checkout_success.php";
if(1 == 0) {
$AuthNet_required = array();
$AuthNet_required[0] = array();
$AuthNet_required[1] = array();
note that in this line:
$<your cart object name>->redirStr = "checkout_success.php";
you need to change <your cart object name> to the name of your eCart object.