The coding is there in the show_menu2 documentation; it just takes some working around. Here's the coding I got to work. My client wanted only select top menu items to not be clickable (links to page ID 4,6 and 57), so I used the following (in index.php of the template):
<div id="nav">
<?php show_menu2(0, SM2_ROOT, SM2_ALL, SM2_ALL, '<li>[if( id==4 || id==6 || id==57 ){<a>[menu_title]</a>}else{<a href="[url]" class="[class] navlev[level]">[menu_title]</a>}]', "</li>", '<ul class="ullev[level]">', "</ul>", true, '<ul id="header_menu" class="menu">');?>
</div>
To disable all top menus where there is a sub-menu you can use the following:
<div id="nav">
<?php show_menu2(0, SM2_ROOT, SM2_ALL, SM2_ALL, '<li>[if( class==menu-expand ){<a>[menu_title]</a>}else{<a href="[url]" class="[class] navlev[level]">[menu_title]</a>}]', "</li>", '<ul class="ullev[level]">', "</ul>", true, '<ul id="header_menu" class="menu">');?>
</div>
I figured someone else might find use for this code sometime.