A/R write offs have to be made to the customers account directly. This can be accomplished by entering a credit to the account, effectively reversing the outstanding payment due. You can use the Allowance for Bad Debts Account for the debit. this will also reverse any sales tax charged and leave a credit in Accounts Receivable. You would then need to go to the receipts module for this customer and balance the write off credit against the invoice to be written off.

Hope this helps