Vodacom's bad service

I took out a contract with Vodacom in May and ever since I've been struggling with having a debit order go off on a day it is supposed to. In June I was debited extra because they said I was in arrears and I let that one slide. Its been 3 Months now having to constantly call and ask when will a debit order go through and this thing is really messing up my credit profile. The agreement was that the money should be debited on the 7th of each however that does not happen and find myself in arrears and having to call and ask what could the issue be. Same with this month when I called on the 8th of July I was told that the debited order method was cancelled and that was now cash of which I did not request to have that changed. Last month in June I had to reverse a debit order because the funds were debited twice due to the same reason that I have to first call for the debit order to go off. I'm really so annoyed and want help with cancelling the contract because my credit record is now a mess because to them and at the end of the day Vodacom will expect me to pay for arrears for something they cant fix. I'm so freaking ****ed off and really want this contract cancelled.