Sounds like the final card to play in the hand could be... "Since you are unwilling to fix this, apparently this is the way you expect your jobs to look. I'll be posting this picture for you, along with your Business Name, so that others can see the quality of your work for themselves."

I had a bit of a run-in with an electrical contractor myself, about a year ago. There were some pretty clear code violations. Larry was good enough to give me the appropriate NEC sections.

It was a fun conversation. I talked with the electrician, his line was that since it didn't fail inspection, they didn't have to fix it. I asked him to call his boss, he said no need.

I informed him that he needed to tell his boss that if they refused to fix this, I'd call the city inspector myself, and make an appointment for him to come and see the work. I'd also work with my neighbors to have several of their homes inspected. After seeing this, I expect the inspector will cut them no slack on future jobs, and they may very well end up repairing the work for free anyway.

I got a call from the electricians boss 15 minutes later. Five minutes after that, the electrician was in my home, bringing items up to code.

Threatening to shine the light of day on contractors mistakes, can lead to increased cooperation ;-)