Ignorance on part of supplier

I had a contract with nashua mobile. They decided to close down and my contract was taken over by vodacom without my conscent. Now I want to cancell my simcard. My contractwith Nashua Mobile (now closed) ended 15 April 2015. I never got any correspondence or invoices from Vodacom. Only received one invoice after a telephonic request. They now want to force me into another month's payment even though I have no contract with them. I want my simcard cancelled with immediate effect which they refuse to do