Angry, disgusted

I wish there was a zero 0 in order to give the rating for Netstar. I had a tracker for the past 3 years then on the 11 March I was informed that the 3 years is up and the contract has ended. We then agreed and arranged for the new tracking device and that someone from Netstart will call me to arrange for the installation (which I am still waiting for that call until today 24 March), and also agreed on the payment date which will be 25th of every month. To my surprise on the 15th March they deducted the money, mind you the tracker is still not installed. This payment was rejected as I did not have funds in my account. Today 24 March I am receiving threatening sms from Netstar that they will backlist me if I don't make a payment, and the amount on the sms is not even the amount that we had agreed upon. Besides the amount being higher than the amount agreed upon, why should I pay or blacklisted for something that I don't even have?