One more image:

I think that's a wedge connector. Maybe one of the pros here has used these. I haven't.

From the picture in the link, I'd guess that you'd have to somehow lever the wedge out with a screwdriver.

If that doesn't work, you might have to go for the nuclear option -- seize the plastic sides with linesman pliers from the open (back?) side, and twist until they break, trying not to damage the wire.

Please let us know what works.

I googled "removing THOMAS & BETTS PLASTIC ROMEX CONNECTOR" and it seems like electricians hate these. :)

But it seems like the easiest way to get them off is use "dikes" to just cut the plastic. I also had to google "dikes" (I was a little worried about what might come up) and it's jargon for diagonal wire cutting pliers.

greg- You must have good filtering software if the first thing that came up was a picture of wire cutters... ;-)

kudzu - tee hee! I'm so pleased you posted because I had forgotten to reply.

DH says you can't call them dikes anymore; diagonals rather:) DavidR, greg_2015, thank you very much for taking the time to look up these and reply. The nuclear option is what worked though the better way does seem to be with the wire cutting pliers.