Hello there,  ChrisD55.

You can create an Expense account named Bad Debts. Create a service item called bad debt and link it to the Bad Debts Expense account.

Create a credit memo for the customer and use the bad debt item. Then apply the Credit Memo to the outstanding invoice.

To create an Expense account named Bad Debt:

1. Click the Gear icon, and choose Chart of Accounts.
2. Click New.
3. From the Account type drop-down list, select Expenses.
4. From the Detail Type drop-down, choose Bad Debts.
5. In the Name field, enter Bad Debts.

You can check this article for the detailed steps and guide on how to write off bad debts. 

Don't hesitate to reach out to me if you need more help.