How to remove large old wire nut
Those are 3M Scotchloks, one of 3M's few bad products.  It was ingenius, it just didn't work as hoped.  Their expertise came in the plastic sheath; however the metal part is designed to deform to hold the wires together.  As such, Scotchloks are single-use IMO.  

Part of the gimmick is the flexible plastic sheath isn't tightly bound to the metal lining.  It's designed to start slipping once the splice is "tight enough" (for some value of tight-enough that I don't trust).   The upshot is you'll spin the plastic part all day; you must grab the thing with pliers to grip the metal part through the plastic.