without details....
solution 1: set page visibility to hidden
if you need on a other place a submenu with the child of this hiddenpage, use the page-id of this hidden page in the show-menu-call like this (43 is the page-id of your hidden page)
<?php show_menu2(0, 43, SM2_ALL, SM2_ALL,'<li><span class="menu-default">[ac][menu_title]</a></span>','</li>','<ul>','</ul>');?>
solution 2 - multiple menus
ckeck the info.php of the used frontend-template for a definition like this
$menu[1] = 'top-main-menu';
$menu[2] = 'sibebar';
$menu[3] = 'footer"';
$menu[99] = 'none';
you need there minimum two definitions, a hidden menu "none" not included
if you have there a definition like this with multiple menus, you can select a non-standard menu in the page settings of this page and this page will not show'n in the main menu
example for the submenu with a new menu-id (2)
<?php show_menu2(2, SM2_ROOT, SM2_ALL, SM2_ALL,'<li><span class="menu-default">[ac][menu_title]</a></span>','</li>','<ul>','</ul>');?>