sprint Charge me after I close the contract boston Massachusetts

I called them and close the contract and told them I want to close all the phones and they didn't close one and I got bill for that one phone of $263.45 and it doenst make sence for me to close my contract and leave one phone, they did it just to take my money and the reason I close the contract was because I wasn't happy with the service and I talk with one of the agent that work there I have the screenshot of our chat that he told me that it is not right what they doing but he can't do anything about it. Since I made the contract with them every month they chrge me extra money and every time I talked to them they come up with excusses.