If they believe you owe them money and clearly they do, they will not go away until the account is paid.
If you pay them what they tell you you owe them then they will pocket your money and close the account.
If you do not wish to pay them any more money then you will have to go through the process described on this site to claim back all the money (Charges and
interest) they have unlawfully taken from your account.
Assuming all of the outstanding balance comprises charges and
interest on these charges, this will clear your account at which point you can close it.