Poor Service. Being ****** by MTN

I took a 40gig contract with MTN but every month they send me 10gig. I have correspondence dating back to January when I was told a call has been logged and that the issue is resolved but when I check it has not been resolved. We are in April now and I have been reporting this issue since Jan. The sad part of all this is that when they debit my account they get their money with no hassles but they can't help me with my issue. How do I cancel this contract because I am getting what I signed up for instead I am being ****** my MTN every single Month since January.