Failure to cancel contract & further billings

We had an account with MTN with a company which ceased trading last year. Despite several requests to cancel the contract they continue to bill us on a monthly basis. I really don't know what to do anymore to get MTN to stop billing us! We should be refunded all charges since the cancellation was first requested in September 2014 as our company no longer has this cell phone or number. You are just taking money from us without our permission and for no good reason! Pathetic service!