Problems with Netstar

I have sold my Polo(DP71KYGP). I Just want to cancel my subscription with them and they said there is a 30 dat waiting period. So now I do not have the car but still have to pay my subscription. I think this is a little unfair. Why do I have to pay for something that I do not have anymore??? This is only another way of making money. I have phoned them twice but they are not interested in my point. Their phone number is :***.