Cancellation issues

I had a 24 mounths data contract with cell c wich was suppose to end on march 2018 i went to the store in johannesburg calton center, of wich i was assisted by a gentleman name Derrick Gadebe who issued me with the cancellation forms that i filled in in the store. However the contract has not been cancelled till now debit order still runs of wich they say i'm currently in erreas for a contract that was suppose to be long cancelled, now they treatning to blacklist me for their fault.