If you think that your customer is never going to pay you for an invoice, you can write it off as a bad debt. However, you can take a look at the Ageing Details report to make sure that you have waited for a sufficient amount of time before writing an invoice off.
To access the Ageing Details report:
Here, you can see a list of your customers along with their outstanding balances and the amount of time that they have remained unpaid.
As a last measure, you can send a final reminder to your customer. To send a payment reminder:
Immediately, an email reminder will be sent to your customer along with the invoice details.
If you still wish to write off the invoice:
The invoice will be written off and the amount will be recorded under the bad debts account.
and GST filing.