When you add a list of sub-page on WP menu, it looks bad when u hover a menu with a list of sub-page. To avoid this we can use some sub page plugins, which will display the list on a page.
To hide the sub-menu by PHP you have to write may lines, but you can do it easily by CSS. This is a very simple trick of CSS for thematic theme. Go to default.css which is on /thematic/library/styles folder. Go to ID #access, make the overflow:visible to overflow:hidden, and that’s it.