main menu subitems

Hi,
i have a main menu and there is several pages under each “category”. (it’s not realy mark as a category, just regular pages).
under each page i have several sub-pages.
when the user hover an “uper/main” page in the menu, i want it to avoid the opening of it so the user could press just the “sub pagese” in the menu.
if you understand my english, please let me know how to do it.
kind regards
Avi