MTN contract

I upgraded my data contract via telephone in August I was on a R500 contact but downgraded to a R149 contract They still bill me at the previous contract amount I sent numerous emails and phoned them thousands of times with still no positive feedback or any results I am now at breaking point I need someone to resolve this matter as my account is rising every month If I knew this was going to happen I would not have renewed my contract MTN is clearly not able to assist They keep on telling me they'll get back to me but never do