Harassed by Debt Collector Even Though They Owe Me a Refund

This company keeps calling me for debt collection even though I paid for a year's membership upfront and in full. Even though I have sent proof of this payment, I still get harassed consistently by their call center agents from the collections department.

Cancelling my contract has not helped because in addition to the above problem, I apparently can't get my refund until whatever internal issues they have are solved.