How to disable clicking parent menu item without disabling it?

The question and the rationale behind it is hard to understand, but "disable menu item without disabling it" I could understand in only one way: it is formally not disabled, but the event of click on it is not handled, or it is handler, but the handler does nothing on certain condition. If so, it all depends on the code of the handler which should detect this situation and do nothing. Logical, isn't it?

However, the whole idea looks like a sign of a bad UI design. No matter how you implement such behavior, the user would not get a visual feedback on this state and would not see that the click on some item is useless because it is "disabled but not disabled". It is not clear what's wrong with normal disabled state of the item.

—SA