Mtn hiking contract package

I was on a 10gb data contract which I was paying for r199. When the 2 yrs was done, I didnt cancel and the next thing I saw a deduction of R320. I stopped the payment as i had not authorised such a high deduction. On querying, I was told its an original cost as the contract had been on special. I feel i am being held at ransom here because even the original cost is not even R320. My number was blocked and am being threatened with blacklisting. Shuld i pay for an amount mtn throws on my lap?