Taking money from my account since cancellation

I opened a cellphone contract with Cell C in 2009 for my mom. I paid my account up to date for the full term of the contract of 2 years. This payment was done via Debit order. I went into the shop and cancelled my contract and moved over to Vodacom. Someone in the shop obviously did not do their job properly as my contract was never cancelled?? Unfortunately I do not have proof or anything that this contract was cancelled. I went through my bankstatements today and unfortunately only noticed the R100 going of my account every month since then. I phoned Cell C customer care and told them about my problem. They told me that I never cancelled my contract and I keep telling them I did cancel this contract. I don't have the sim card or phone or nothing anymore... I am losing a lot of money for nothing and obviously I am not happy!!!!