How Do I Prevent a Page From Appearing in the Menu?

what wordpress are you using?
and what theme,
if you are using the pages as a widget than there is a option to exclude any page in 2.5 and above

please theme, what wordpress tx

you can also check out this plugin
http://realespace.com/2008/04/04/excluding-pages-from-navigation-menu-in-wordpress-update/

this is also a web site that has directions on doing it
http://theclosetentrepreneur.com/excluding-posts-and-category-specific-pages-in-wordpress

Reading the docs doesn’t hurt either…
Template_Tags/wp_list_pages#Exclude_Pages_from_List

most of the time you want to give a solution that is not modifying code fist