Bally Total Fitness Failure to uphold cancellation policy St. Louis, Missouri

Seven years ago I moved from St. Louis, Missouri to Litchfield, Illinois.  Due to a clause in the contract, I legally separated from Bally Total Fitness.  A couple years later a collection agency contacted me saying that I owed the balance on the contract.  I was able to produce the letter after locating it in my files.  Now 7 years later, I received another phone call from a collection agency stating that I owe.  This is harrassment!  Am I to receive a call every couple of years and be required to produce a letter that was issued to me 7 years ago?  When is this going to be over?  I feel very harrassed and angered.  I don't intend on sitting here and "taking it".