Unfair treatment and poor service

I am very disapointed on their service . Truworths sold my account to sigma and they collect on their behalf I settled my account last year 26 March 2019 but im still listed as owing them . When diaputing it to them and sent proof of payment they never get back to me and my credit score is bad bcz of them when applying for finance i get declined saying I owe Truwoths they dont remove people on the buero . I want them to remove my name coz I have paid them last year and I have proof.