Bootstrap 3 has built-in visual support for disabled dropdown menu items as seen from their dropdowns.less file:

However, these are simply visual changes and don't actually impact the interactivity of the element itself, meaning it will still be clickable!

We can get around this by utilising a combination of the [disabled] attribute (for oldIE support), and the CSS pointer-events: none rule (for evergreen browsers), a technique outlined on StackOverflow. Here's the CSS (or LESS, SASS etc.):

Once added to your styles, you can easily disable any dropdown item by applying a [disabled] attribute to the <a/> element itself, and adding the usual .disabled class to the parent <li>, as seen below.

And that's it! Your links will be truly unclickable. This will work on all evergreen browsers, as well as IE6+, though may have limited support for older versions of Opera. For more information, see the StackOverflow post.