Exceptionally bad service from MtN

I took out a data contract with MtN but within the first year the price had changed three times and then my package was changed due (to a smaller one) because of my so-called 'high usage' despite me being up-front about what I needed it for and only once going over the 'fair usage' allocation. I subsequently cancelled my contract with MtN six months ago but am still receiving bills for it despite numerous calls, emails etc. I am also getting threats of negative impacts on my credit score.