Bad customer service

I have been with Rsaweb for the past two years on a month to month basis. Two weeks back they send me an email to inform me that I owe them R4000 as my payment has not gone off for the past few months. I left the money there and my thing is that I am on a month to month contract with them so when the payment did not go off the first month then they should have disconnected the service and I would have made a manual payment. I am so disgusted by this service and want them to come and fetch the stuff because I am not gonna pay that money. It is not my fault that they let it go on and add up like that. I will rather find myself a new service provider.