Request to add a limit not executed

I received a call from Vodacom saying that they would migrate my account to another package. I accepted but did not understand that this was not a top up package but a contract without limits. For 2 months the data costs on the account was very high so I phoned the call centre and requested that they place a R100 limit on the account so that we can only use the free minutes. I just checked the account now and there is another R1000 data charge on the account which means the limit was not placed on the account. What do I need to do to get this fixed?