No Signal, No service - but paying for contract

Signed a 2 year contract. Received the phone and card. Arrived home and realised that the Cell C signal is really really bad. We live on a farm and asked them to check it before the time. We were told that there is no signal problem in our area. We can not use it at all. We complained and we were told that they will come to our farm and see what the problem is. We were told that a booster might be needed and that cell C will do it at their cost. We've complained, requested their attention and visited the store many many times. We finally requested for the contract to be cancelled. We've completed a form in the Rustenburg Waterfall Mall branch. The debit order is still going off our bank account every month. The only usage on the contract is the times when Cell C store in Rustenburg were testing the phone or were download new software to see if the phone will not work if it is updated... I can use it in Rustenburg but we do not go to town that often. We own a guest farm 40km from Rustenburg on the R30 Derby road.