Had you tried depth argument (see wp_nav_menu() documentation)?
Also do I understand right that you do not setup menu manually and let theme generate it with custom callback that you have in your code?
Update
Same answer – try depth argument, only this time in wp_list_pages() call in starkers_menu() function. It should work, unless that function is never processed and not available at the time of menu running.
Do you use latest version of theme?.. If not consider updating.