Telkom data Contract

We have a data contract with telkom with a route and advise them once it reaches it limited it must be cut off which they agreed. The first month cost was very high , which they said they was going to investigate which they never did, the second month the same thing happen with the same excuse. We decide to take out the route and the amount still came high. We called them and same excuse if given but they said the first 2 cases which they was investigating was closed with no out come. After numerous calls they have said someone was using our line which i think is impossible .