Recently a question arose about removing layers globally from a linked AutoCAD file inside of Revit.  The user had went into Object Styles, Imported Objects Tab and then expanded the file for the .dwg file then proceeded to delete the desired subcategories for object styles.  This sounds like a completely legitimate way to go about this, with any Revit category if the subcategory is no longer needed then we simply delete them.  In the case of a linked AutoCAD file this is not the right way to go about this, the end result will delete the undesired layers but all of the arcs, lines, circles and text will remain in the file. I found this interesting and a bit of a surprise.  If those elements in the CAD file are queried  the will be un-associated to a layer therefore making it impossible to remove them out of the Revit file.  Instead select the CAD file and use the Delete Layers option, this will remove the layers AND all of the associated elements form the entire file.