Angry customer complaint

I have data contract with mtn,all this years they were debiting what we agreed on.this month they debited me almost double the amount agreed upon on my contract,I called them n asked them they told me the other money is for contents services which I know nothing about. The told me if sum1 is connected to my router and that person subscribe on something they charge me which is totally wrong because as the service provider is there job to block those kind of thngs.as a customer I can’t control what other people do on internet while connected to my router is totally impossible and is unfair on me as the customer as there is no explanation on my contract about any content services and is not my first router with MTN nothing like this happened before.is not my responsibility to keep taps on what people do on internet. I paid the amount I’m supposed to pay at the Davenport mtn shop. I received sms this morning saying to stay connected I must pay the remaining amount of the content services which I told them when I called them I will not pay for something that is not on my contract.