Poor service from mtn staff

I'm so angry and disappointed with MTN, Ive been using it for so many years, this year January they sold me data contract while the other one was active, the guy who assisted me told me to cancel the old one and it's possible, when I tried to cancel for the first time the guy working from mtn advised me with a better deal for the same premium, my problem now is when I want to cancel I get different stories, I'm no longer using the Sim because it was lost and I'm unable to cancel, I'm now paying for two useless contracts, today the lady told me to pay the cancelation fee together with my next month premium, so I don't know what to do now,please mtn do workshops for your staff, I no longer need that contract but I'm paying