Poor service from Vodacom

I forwarded an email a month prior to cancel a contract that ended 30 March 2020, however, contract was never cancelled and i am now liable to pay another month. I refuse to pay as i cancelled in time. When i called to enquire i was informed that they only cancelled on 08/04 after i sent a follow up email to them. This is unacceptable. I therefore expect this matter to be resolved with immediate effect.