I want to delete a category from Imported Objects tab in Object Styles.
Autodesk wikihelp says:

You can delete any user-created object style.

Click Manage tabSettings panel Object Styles.
Select the category name to delete.
Click Delete.
When prompted to confirm the deletion, click Yes.

But when I select the category the delete button is disabled and simply hitting the delete key does nothing. I am suspecting that Revit thinks the category that I am trying to delete is not user-created, but the category I am trying to delete is a result of a dwg file that I imported.
Revit does allow me to delete the sub-category but never the category. Any suggestions would be appreciated.