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Goto Manage > Object Styles > Imported Objects >
and then you should see each drawing that was imported
delete the layers from each individual drawing and then you should be able to purge the drawings out.
Google brought me here. Thanks for the suggestions guys! My Revit however didn't remove these imported dwgs from Object Styles or VG even after I manually deleted all of their layers and then Purged Unused several times!
For those still looking for an answer, I eventually found it at the end of this thread: https://forums.autodesk.com/t5/revit-architecture-forum/deleting-imported-categories/td-p/1455947
Hope this helps