ok, found the answer here : http://codex.wordpress.org/Function_Reference/wp_list_pages#Exclude_Pages_from_List
here is the core of the answer :
Markup and styling of page items
By default, wp_list_pages() generates a nested, unordered list of
WordPress Pages created with the Write > Page admin panel. You can
remove the outermost item (li.pagenav) and list (ul) by setting the
title_li parameter to an empty string.