User has created some write-off payments and these transactions are getting logged against the Default Item Expense Account. But the user wants to specify a Bad Debt account for these transactions and has already created an expense account for this.
To log your write-off payments against a specific account instead of the default one, you need to select that account as the Posting Account when creating the payment. In the Add Payment screen, when you select the payment method as Write-Off, the Posting Account drop-down appears. Select the account that you have created for logging write-off payments (Bad Debt) from this drop-down, apply the payment and save the changes. The payment gets logged against the specific account. Check the Core Help for more on creating write-off payments.