Could someone help me find a rejection format as far as refusal to pay a debt?

Last year, I was scammed for $600 by a "traveling agency" that was only suppose to charge me $150 for the 1st month's membership unless I called and canceled within a 30 day time period after receiving a note. I called and canceled within 30 days but money was still taken from my account using an electronic check of which I previously provided the information in my ignorance. I recently just finished paying off the debt owed to the bank, but on my credit report it is saying I still owe the company $150 and they hired a crediting agency which is calling my phone asking for installments because these people are arguing about how they want their money. I explained to the representative in the agency what happened and they told me I would need to write a letter explaining why I refuse to pay the debt. I've been trying to research an appropriate template or example I could use to guide me in writing this letter but I keep coming up dry. Can anyone give me any suggestions?... Thank you, all of your istants is GREATLY appreciated.