The layers can be removed only when the object resides in the layers will be removed, once it is removed it is an empty layer. The empty layers cannot be deleted by purge. It might be because the layer is frozen on a viewport or referenced by an object in a block definition. By using EXPORT command it is possible to remove the layer from drawing, which results in creating a DXF file of the drawing. You can edit the dxf file in a text editor and rename all the instances of layers in the file, except the layer definition.