SICK AND TIRED OF MTN!!

I took out a cell phone contract with Nashua Mobile in Jan 2014. At the end of 2014 I was informed that MTN was taking back there cell phone contracts from Nashua Mobile, and that from Jan 2015 I will be debited by MTN.<br> My contact is a top up acc. which I pay R169.00 a month for. in Jan I was debited R198.00 which was incorrect, I phoned MTN customer care line and asked why this happened, they said it will be credited on my next debit. Feb I was debited R169.00 which was correct but I was not credited the R29.00 from Jan. I phoned again and they argued with me that it was credited, after a long conversation they said it will be corrected in March. in March I was debited R140.00 which corrected the incorrect debit in Jan. April I was debited R181.00 again I phoned and emailed as I was not getting any joy from the customer care line. this time they said that it was Nashua who was debiting me incorrectly (which I don't get as MTN now debits me) and I received a statement yesterday saying my debit end of May will be R195.00. I am fed up! I have emailed them again asking for a form to cancel my contract, as I no longer wish to have anything to do with MTN! still no answer on from them.