Contract cancellation

I wrote Tracker SA an email by 18th requesting cancellations and termination of the contract stating very clear how disappointed I am with their service. Today I received a call that I still have to pay for September as contract is for 1st of every month, mind u I have been paying on the 15th. I am crucified like this because THEY have already generated Sept invoice. I am tired of this people and their call harassment because when I needed they calls I was never assisted. Please help I am desperately in need of away to move far away from this people. Mind you little did I know I was on month to month contract from onset, I should have moved long time ago