Lawyers claiming money from an account that was settled

I have a truworths account and it was in errears. In April 2024 i got a call that my account was handed over to RL Darly i called them and arrange a payment. I paid the account and settlement letter was sent to me. I asked Darly to close the account but they didnt. Now i am getting messagaes from Nimble lawyers saying that i owe them money and that Darly has handed over my account to them. How can you hand over an account that was paid up and settlement letter was sent.

Can you please close my Truworths account, I dont owe it anymore.