Forced Data Sales

I am very angry with MTN, on the 21st of October I received an sms saying that my account has been credited with 4 Gigs of data, i then asked sister if she bought that for me and it wasn't her. a few days later i get message from MTN that i owe R405 and my my outgoing facilities were blocked, i went to the MTN Midrand branch and I was told I did a self service and i need to pay that amount. I did not do a self service and i dont know how that works, my line kept on being locked and i went to the Rivonia branch still no one could help with this. MTN kept on sending threatening messages , i called the call center until i was told that i used the data therefore i need to pay for it, the amount keeps on changing and its now at R363.50. I pay my normal monthly subscription, I refuse to pay for an amount that i know nothing about and that forced on me. please assist because this is getting out of hand. MTN is de*****ing their clients. i did not sign up for these extra amounts that keep keep coming up. please assist. This is forced sales and i refuse to be part of it.