VERY BAD BAD SERVICE

I closed my account in August 2020, sent all the proof and I got a confirmation letter stating that the account is closed. It's now Feb 2021, customer service has not been so useful and ensuring this account is closed and this is affecting my credit score as it still appears as active. I have called countless times and I am still getting statements. What will it actually take too ensure that the account gets closed, this has ****ed me off now for the longest time because it clearly shows how inefficient your accounts team is and customer service. Who can I actually escalate this with because I am tired of getting reference number without an actual resolution.