MTN is rotten to the core

I had a router contract with MTN for a period of 24 months. Prior the end of the contract I went to their stores in polokwane requesting that in the 24th month I won't be renewing the contract instead they changed my package upgraded it and suspended my sim .. on the 2nd month I got SMS saying I owe them R614 I paid it out of frustrarion and demanded that I get a paid up letter. I've since been trying to get hold of them they ignore my text even in their branches they give me nasty attitude whenever I go ask how far are they with the paid up letter. I am tired of MTN for robbing us day light like this. This can't keep going on like this. How do you expect me to pay a service that I haven't used