wp_list_pages hierarchical argument not working?

try this

<?php wp_list_pages('title_li=&depth=1' ); ?>

I have the same problem – subpages showing in my navigation bar – so where do I insert the code to change the depth? Thx.

If I understand how this arg works, it should hide the listing of submenus. right?

No, absolutely wrong.
Read the parameters’ description: Template_Tags/wp_list_pages#Parameters

I had the same issue after a bit of research settled for this:

wp_list_pages(‘title_li=&hierarchical=0&depth=-1’)

depth= -1 passes the parameter to display all pages in a flat non-hierachical order.