Newspaper clips
If none of the other great suggestions posted here work for your superiors, then I would suggest printing out newspaper clippings of other medical professionals who tried to cheat the system and were busted for fraud. I am sure there are articles out there with the same scenario. Seems like every other day here there is some type of medical frauder caught.
That should get their attention, but if not; then report and leave. If you do not, and say nothing you could end up losing your certification. So not worth it.
I too, went through this when I worked for a DME company. I tried numerous times for them to see clear of incorrect coding and they refused telling me to drop it and they ended up taking my file away. I was the appeals and denials coordinator, but I was also a CPC-A at the time. I and another employee reported them to Medicare for fraud. Within a year, they were arrested in the 2010 Medicare Strike-Force Sweep across the nation. Just this year, court proceedings were finalized and both owners found guilty on numerous counts of fraud.
Keep your head up and stay positive. Believe you are doing the right thing when they will not listen even when you have shown them verifiable proof. Even if they don't take your word for it, they could call someone and verify the information.