mtn bill me as they like

My contracts whas with Nashua never had a problem . But they closed last year and it went over to mtn . I am complaing to mnt sin desember 2014 my contarcs is top up contracts but mtn charge me as they like I paid every month and complaint every month till end April I refuse to pay till they sorts outstuff . They told me they wil gave me the credits because I paid to much since desember but today they again told me my account is over R 3000 I'm so tired of mtn