I use a pair of Copper wire cutters, they are very similar to Garden loppers, they have two curled 'beak' like jaws that over lap.
I posted a pic of then yesterday. 'Marvel' brand though.
Two chops and the wire falls off almost. The Ferrite does crack a bit, but that makes it easyer to slip the Copper wire off.
Once that's done, I can dust off the ferrite dust, then pick out all the loose Copper wire and what's left is the Copper wire that's stuck together with any varnish that was on the wire.
I can then cut around that varnish with tinsnips to get the clean wire off.
The patch of varnish with one layer of wire gets put into the 'plastic coated Copper wire' bin.



I don't end up with ferrite crumbs in the wire and almost no varnish either.