public void DeleteAllUnused(Autodesk.Revit.DB.Document document)
{
   using (Transaction transaction = new Transaction(document, "Delete unused elements"))
   {
      transaction.Start();

      ISet<ElementId> unusedElementIds = document.GetUnusedElements(new HashSet<ElementId>());
      while (unusedElementIds.Any())
      {
         document.Delete(unusedElementIds);
         unusedElementIds = document.GetUnusedElements(new HashSet<ElementId>());
      }

      transaction.Commit();
   }
}

public void GetUnusedElements(Autodesk.Revit.DB.Document document)
{
   // Get all unused elements in the document
   var allUnusedElementIds = document.GetUnusedElements(new HashSet<ElementId>());

   // Get unused elements without a category
   var unusedElementIdsWithNoCategory = document.GetUnusedElements(new HashSet<ElementId>() { new ElementId(BuiltInCategory.INVALID) });

   // Get unused wall and floors types
   HashSet<ElementId> categoriesToPurge = new HashSet<ElementId>
   {
      new ElementId(BuiltInCategory.OST_Walls),
      new ElementId(BuiltInCategory.OST_Floors)
   };

   var unusedElementIds = document.GetUnusedElements(categoriesToPurge);
}