No luck with that one. I'm not part of a HomeGroup and they still show up. But to try it, I turned HomeGroup back on and then tried to remove the Sonos device but it tells me it can't remove it because the device is online and that I should turn the device off.
The Cisco STB didn't even show up under the HomeGroup devices.
I right-clicked on the device where it showed up in the file explorer window (not in navigation pane) and clicked remove on both and now they are gone. I rebooted and they have not come back as of yet. I guess we'll see if they come back on their own.