What you actually need is to have the link for the parent set to "#" so it acts as a placeholder. I haven't done this before, but I can give you some information that may make it easier for you to figure out how to do it.
As usual with WordPress, there is more than one way to accomplish what you want. One way would be to create a child theme (so your changes aren't over-written when you upgrade WordPress), then add code in functions.php that would add the "#" for the specific menu items you wanted to be placeholders.
An easier way would be to look for a plugin that would allow you to do what you want. It looks like there are several candidates, though some of them I looked at look like they aren't actively maintained. There's one called Page Links To that is apparently actively developed, works with WordPress 3.4.2, and lets you set the link for a page to whatever you want. With the Page Links to plugin installed, you could just set the page you want to use as a placeholder's link URL to "#" (the number sign without the quotes) and you should be all set.