Netstar Altech poor service

I had a three-year contract with Netstar, after the completion of the contract i wanted to terminate my contract, i contacted netstar in April, asked one of the reps to cancel, she promised to cancel but never came back to me. In June i wrote a letter/email informing them that i would like to terminate my contract but they still ignored my letter. Just a week ago i received calls from a company called creditwox, asking me why i did not pay my account, when i tell them that i cancelled my contract with netstar they asked me to send them a letter of cancellation, i emailed it to them, they also never came back to me. NETSTAR refuses to cancel my contract even after i completed my three years with them, they even told me that they will include arrears, could you please ask these people to accept my cancellation, out three years contract is finished a long time ago (Nov 2017)> my account is ********** 9