BuddyPress.org

You can use bp_core_remove_nav_item( $parent_id ) to remove items from the navigation array. $parent_id is the slug of the parent navigation item. So, for example:

function my_nav_item_removal_function() {
    bp_core_remove_nav_item( 'friends' );
}
add_action( 'init', 'my_nav_item_removal_function' );