Allowing usage when it was blocked

I personally went into Vodacom at Northgate to block the usage of data to only allow an amount of R50 over the limit. Last month the billing was correct for the January usage. In february they allowed the usage to go over by R406 and say that it is not always guaranteed so I am liable to pay this amount. With this particular account we have experienced numerous problems in this regard and that is why I eventually cancelled that contract. I am extremely annoyed about this and feel that Vodacom did not offer the service requested.