Is there a way to set up certain menu items up so that when the have submenus, users can only click on the submenu items and not the main item on the menubar?

You can't make it change automatically, but you can certainly configure it as indicated (on the Page Settings).  That would look like this:

Some folks also like to have that menu item simply default to the first page in a submenu... so rather than be completely "non-functional", it just leads to the desired location.  You can easily do that by changing the TYPE of link to Page and specifiying whichever page in your site is apppropriate for you.  That setting is also on Page Settings at the bottom and looks like this:

Cheers!

Scott Willhite, Co-Founder DNN