Top level menu question

Hi,
I have several items on my Top Menu. Each contains several 2nd level menu entries. I’d like nothing to happen if the user clicks on the top level, thus forcing them to click on one of the sub level entries. I’ve seen this on other web sites. How can I set this up ?

On the top level, I have a category, under which I have several pages.
Thanks for your help.
Thanks,
Steve