I've seen this request before but I don't understand the reasoning behind it.
If you don't want to show the menu items, why have them in the first place?
If you want them to show in a split menu fashion, why not just make a new menu for that part and have it show on the certain assigned main menu item pages that you need it for?
My website has many sections, each one having many categories in turn. This result in a menu with too many levels and lots of links.
I want to avoid this behavior, and I also want to keep the structure of my joomla menu intact because I use it to show all menu options in a left menu that changes from section to section.
Having several joomla menus would result in a higher level of complexity and extra difficulty in managing the joomla installation.
This way I only have one joomla menu with all items I need, and at the same time I have less unnecessary links in each page of my website.
Now the change is done, you can view the result here:
http://www.gretur.com
, with an unique joomla mainmenu, an unique menu module in the left, that is the same module wichever section or category you are, and a rokmoomenu limited in the number of sublevels it can show.