Lady of America Fitness Centers calling me when I owe them nothing

This company has continued to hound me even though my contract has expired and been paid in full, I contacted them on the phone approx. 3 weeks ago and requested a letter from them stating the status of my account to accompany the final payment of what they stated I owed.

I believed the contract to have been paid in full, but decided to pay it IF they would send me a letter stating that my account would then be closed just so I could get this resolved and be done with it.

They sent the letter, I sent the payment, copied the letter, and mailed it by Registered mail.

The same day I recieved the pick-up confirmation, they are calling me again!!! I have not returned the call, however I suspect they are now going to claim I owe them something else. I want these people out of my life, and I don't want them making fraudalent claims against my credit, judging from their past behavior-I would expect that next.