Unfair Billing for slow speeds

I had a crystalweb account for my sons for gaming in 2015. Speeds were so slow that they couldnt even play a decent game even when we had a 10meg line. I phoned in to cancel and it was confirmed telephonically that the would speak to the accounts dept. I signed up with Afrihost with the same 10meg line and immediately received fast speeds for gaming and downloads. Then got a call from crystalweb to write a letter of cancellation , which i did in december. However now in March 2016 , i get a call from their accounts department that i owe R549 for the whole of December ( for a few megs that we struggled to use !! ) The accounts lady threatened to escalate the unpaid account to their legal dept if not paid. I have since paid the amount reluctantly , just to avoid endless legal problems. But this is to warn other people to watch out for crystalwebs slow speeds and then still expecting us to pay for it irrespective . BAD SERVICE when the same line used with Afrihost was working very well.