Hearst Corporation Country Living Magazine repeatedly sending me bills for magazine subscription I have already paid for Harlan Iowa

I paid for 10 issues in June 2012. I was sent an invoice for subscription services/renewal notice beginning early this year, 2013, which I ignored because I had no plans to renew. I continued to get invoices in the mail, so I sent one invoice notice back to them with "cancel" written on it and "I am not renewing".

Then I began to get letters attached to the invoices that state "We have sent you numerous bills. We must now insist that you send us your payment immediately" and "this will be your final opportunity to restore your good standing".

I wrote to them requesting clarification about why I owe them money for a magazine that I am not renewing and I attached a copy of my cancelled check showing that the 10 issues I received had been paid for. 

I received another invoice/letter from them yesterday, same wording as above. It is signed Yours Truly, Country Living Customer Service

There is no phone number given to call, there is no name given as a contact person in the subscription services department.