Getting summoned for contract expired

I used to be a Nashua mobile client and this account was given to Vodacom. It is for Internet use. I signed a 24 months contract with Nashua mobile. This contract expired February 2015 already. In May 2015 I phoned Vodacom and asked to cancel the contract, because I don't want to extend the contract. They actually owe me money. In December I received a text message saying that I need to contact lawyer firm. I did and explain the situation, I also send an affidavit, 2 days ago received a text again that I will be blacklisted. Unacceptable! !!! I didn't sign anything with Vodacom.