re: Refusing to purge
Hmmm... there's a political and/or eating disorder joke in there...
Anyway: layers nested inside imported blocks are always my first suspect. I've never worked on or with the kind of documents you generate, but if you use a library of third-party blocks, or import drawings from other sources (as I do, from architects and equipment vendors*), that's the first place to look.
[* - Steve may agree with me: drawings from food service consultants are the WORST!]