CELLPHONE CANCELLED

I'm very upset with Sprint PCS. I bought two cellphones in Katy Mills store and I go back to store in less than 30 days to do a devolution. The salesman that attended me was not there and the manager said that service could be cancelled the day after (because he was not there) and that I will not have a problem with the date. The service was cancelled after the limit date (30 days) and I am receiving letters from Sprint charging me. I went back to store again, talked with salesman Dante, he said that the problem was solved, but I have been receiving a lot of letters again. The problem is not solved. I went back to store and Dante said that his manager said that he can't do anything. They have caused me so much stress since September last year.