Hey Ron. I'm not sure about something specific to ArcFM, but in general it is possible to script the process of removing custom class extensions. There are a couple of examples in this Esri knowledge base article: Error: Error opening feature class. Unable to create object class COM component
If you take a peek in there, maybe there's something that will help you out.
No scriptable solution I'm aware of but this thread on GeoNet may provide an ArcObjects solution. https://geonet.esri.com/thread/33483. There is a zipped add-in right at the bottom of the post. I've been working on an ArcFM Geoprocessing toolbox to make administrative tasks like adding/removing model names, autoupdaters, etc scriptable through Python. A tool to add/remove the ArcFM class extensions is definitely on the list but haven't got to it yet.