MTN cost *******

I have been billed for past 6 months for content charges at around R200 per month. When I queried about it I was told to check for active subscriptions by dialling *155#. Did this. No active subscriptions can be found. MTN customer service initially said there is nothing they can do but they continue to bill me monthly for this. Nobody can tell for what it is and nobody can stop it but they continue to bill me.