Depending on the setup of your phone, go into Apps and Disable/Force Stop all MTN related apps you aren't using. For example, on my one phone, I've disabled "MTN DS", "MTN PriceCheck" and "myMTNza".
Do the same for Cell Broadcasts - not sure if this will work, as I've never received an MTN pop-up, but doing so once helped me out with stopping a nagging voice prompt.