I know this is super late but I was just messing around with it trying to get rid of the same line with the device playing, as you were. I figured it out, so I figured I might as well post it here since maybe you still want it, or others may want the info too.
You need to edit the file NPOS_DomBuilder that is located in the MMM-NotPlayingOnSpotify/core folder. Line 97 should say:
content.appendChild(this.getInfoDiv(’ ', context.deviceName));
This is the line that displays the info we want to get rid of. You can probably just delete the line and save the file, or just comment it by putting // at the beginning of the line. Hope this helps someone.