Sometimes layers get embedded into objects or blocks.
I usually will create a backup of the dwg and start deleting large sections of it, and purge each time, until I can identify the object that had that layer on it.
Once you've identified the problem child you can fix it.
If it still persists in the dwg after doing that then I will WBlock the whole dwg out to another file.
That used to get rid of some persistant problems.
Hope this helps. Let us know what worked.