Write a letter to the President of the corporation and mark the envelope, "Confidential." Include a clear description of the problem and how you have been treated. Chances are fairly good that one of his underlings will pass it to someone with the power and motivation to straighten it out since it comes from the "Office of the President."
This is what worked for me when Citibank messed up after I paid off my student loan early.
This is what worked for me when Citibank messed up after I paid off my student loan early.