Sometimes there is a layer that you just can't get rid of.  Here's several tried and true resolutions for deleting those pesky layers!

First, open the properties box and use the Quick select filter button to find all entities whose layer equals the one you want to delete.  This usually finds any objects on the problem layer and lists them right there in the Properties box.  Simply change the layer properties of the chosen entities or decide that you do need that layer for those objects.  If no objects are selected don't forget to look in the layout tabs for objects.  The quick select filter dialogue will not find objects in paperspace if run from the modelspace tab.

The second option is fairly common.  You have an empty block definition on the problem layer.  use the Erase Blank Block PostalCAD utility to delete the definition and then use the Purgeall.

The third scenario comes with a big warning, as does the command line, when using this method.  A deeply nested object within a block definition usually is the culprit here.  Draw a line on the problem layer.  Go to the Express menu and under the Layers cascade issue the layer delete utility.  Select the new line with the correct layer properties and prepare for a stern warning at the command line. 

******** WARNING ********

You are about to permanently delete layer 2 from this drawing.
Do you wish to continue? [Yes/No] <No>:

Type in Y for yes, if you are absolutely sure there are no objects on the said layer.  Should there be an object on that layer, you will be told via the command line what the object or objects are.  Typically there are no objects, the layer is corrupt, and the effected layer is deleted from the drawing.  Don't abuse this utility!  You could very easily delete some needed information.  Always use the Saveas command before using it.  Also, use the Layer All PostalCAD command beforehand to visually show all layers or check the drawing using the Layer Viewer.