How to disable permalink of a particular menu item?

How about just go to the menu from wp-admin remove product menu(i.e the current one) now create a new menu (Product)using Custom Menu and put url to # and add ur sub menus,
if u have different class for current menu u can add custom class too follow these steps
In Appearance > Menus, click the Screen Options tab.
Under Show advanced menu properties, check CSS Classes.
Now expand any menu item to reveal the CSS Classes (optional) text input