Cancelled my contract am still.billed

I am furious.I cancelling my contract in April following voice prompt.no action was taken.ask whats the point of that self service to cancel.I followed up in May and specifically gave instructions.Call centre agent assured me that it was cancelled bt still no.They keep on billing my acc and giving me airtime for two months now.I am not going to pay for that i never used it.And if Vodacom dare list my.name as slow payer after 24months which i never skipped.payment. they will regret this. they shouldask MTN abt me.