So the first thing to do is to call them and ask them to admit that they have a copy of the letter you sent. Bluff them and tell them that you sent it by certified mail so you know they received it, and ask them to confirm.
If they do confirm, then ask for a confirmation of the receipt of the letter to be emailed to you. Then, tell them that you would like to record them confirming on the phone that they are confirming they received the cancellation letter.
Then you would send this information to the debt collector and demand that they immediately halt all debt collection
If they do not confirm and deny, then you are going to have to actively dispute that you owe anything with the debt collector. The way you do this is you send a written denial of the account and demand that they do not continue to contact you in any attempts to collect the debt unless they are willing to sue you on the debt. Copy this letter to the three credit reporting agencies. Their contact information will be found on the following page: http://www.usa.gov/topics/money/credit/credit-reports/bureaus-scoring.shtml
If they do not drop the claim, then you may file a lawsuit against the debt collector in small claims court for violations of the Fair Debt Collection Practices
Act and seek monetary sanctions against them (if you want to handle it yourself).
If you want to hire an attorney, he/she can file the debt collection act case in a higher court and have both sanctions and an injunction entered against the the debt collector, and a declaratory judgment that the contract was cancelled. Because it is a contract case, you can win an award for reimbursement of any attorneys fees you've paid out to file the lawsuit.
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