Deleting Object Styles Category

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.

Subcategories under the Imported Object tab are not considered "user-created". You will have to located and delete it within the imported object (i.e., delete that layer in a .dwg import via AutoCAD). The only Subcategories that you can delete are those in which you created from the Object Styles dialog. This includes user-created subcategories that are imported from loaded Families.

Also, the dwg is no longer in the model.

The appropriate import category should have been removed along with the import, then. I just tested in Revit MEP 2013: imported a .dwg and then deleted it, and the categories vanished. Are you absolutely certain that all instances have been removed from the Project? Have you made sure that it doesn't existing in other Views, such as Drafting Views?

Could it be that known issue (in older RVTs) related to Groups? http://ideatesolutions.blogspot.com/...ting-dont.html