Fee dispute

My Contract with Virgin Active had expired and I was on a month to month. I missed a payment on 25 November 2016 and They immediately took my access away so that I do not enter and use their facilities. I cancelled a contract and I was told that I owe them a monthly fee which I disputed that and I do not have a contract with anyone since my contract the I signed has expired. They handed the account to some agents called MBD and now they claim that I owe them More than R4000. 00 and I do not owe them anything.