As of 2.1, from bootstrap documentation at http://twitter.github.com/bootstrap/components.html#navs, you can.
Disabled state
For any nav component (tabs, pills, or list), add .disabled for gray links and no hover effects. Links will remain clickable, however, unless you remove the href attribute. Alternatively, you could implement custom JavaScript to prevent those clicks.
See https://github.com/twitter/bootstrap/issues/2764 for the feature add discussion.