The Exclude Pages plug-in works well for me in TwentyTen.
matius, did you find a solution?
You may be able to write another CSS rule specifically targeting the Flexi widget pulldown menu that re-applies the display:block.
mikelite, you can find this code in wp-content/themes/twentyten/style.css on line 430.
It is better to make a child theme and edit the CSS there, however.
This is tagged as resolved but there is no solution provided. If the original poster has already found a solution, please post it here to help others with the same problem. Thanks.
In the meantime, this thread may provide a solution:
http://wordpress.org/support/topic/top-page-nav-menu?replies=11
