Unselectable value in a HTML list

I j开发者_Go百科ust discovered this feature in Gmail. In the list where you mark your mails as read or unread, I noticed that there are some values (like "Apply label:", "Remove label:", the separators etc) that cannot be selected. I mean if you click or select them, the list doesn't even close up.

Can anyone tell me how that is achieved? Is there an extra attribute? Javascript? CSS?

I am thinking of doing the same thing to values like "Select month" in a list.

Complete rewrite in response to comments:

The options are just disabled