It would appear that deleting all of the layers for an Imported object purges the drawing out of the Revit project file. I checked the Visibility Graphics tab for Imported Categories and when I deleted all of the Layers including "0" the DWG was no longer listed.
What is more critical is when you delete all of the layers in an imported DWG, except those have no objects on them at all. Then try to purge or delete it from the Revit project file. Since no entities remain on a layer or are visible with which to select the imported DWG, you cannot delete it from the Revit project file.
The answer was found in a thread on AUGI's Revit forum "Lost Import"
To get rid of any remaining layers from an imported DWG without entities. Open Visibility Graphics, select the layers in question and pick the Object Styles button at the bottom of the dialog box. In the Object Styles dialog box in the Imported Objects tab select the layers and Delete them.
But this does not remove the imported DWG file from either the Object Styles or Visibility Graphics. Which suggests that the imported DWG is still in your Revit project file.
I have not found the answer to purging the imported DWG file in the above scenarios