Contract was cancelled in April 2015

My cellphone contract term ended on the 28th February 2015 and I cancelled the contract on the 30th April 2015. I have send virgin mobile mails and called them to cancel the contract and still thy tried to deduct the amount from my bank account up to August 2015 when I send them another mail. Today a debt collector company called me and said I owe Virgin mobile R1300 and something rand. I cancelled the contract and still they don't cancel it on their system.