The best way to do this is as follows.

If you create a new line item ( a placeholder ), and name it "unpaid debt" or "written off" and add that to the invoice as a negative amount. 

So that the invoice will be "paid" and you will have a record of all "unpaid debts".

Here are some other options to manage bad debt customers.

1. Tags, you can tag that customer so that you know that they owe.

2. Credit limit, you can set a specific credit limit for the customer. If they reach that limit you will be notified that they owe.
3. Payment type, so we can add a payment type for you called "write off" so instead of choosing cash or EFTPOS you would choose to "write off" the debt.