It seems that you'll just have to bite the bullet and pay it.

My dentist once messed up my insurance filing.  They got the insurer's name wrong!  And the claim of course was rejected.  The the dentist office didn't let me know for a couple of months, and just classified it as unpaid.  And they almost reported me to the credit rating agency for late payment!

They lost my business for sure.  But my husband still goes there!!!!