You could refund the balance to somewhere like Petty Cash, and then tag it to any nominal code you wish (using the ‘Something else not on the list’ option).
There’s a guide on refunding the credit to a bank account here.
Disclaimer: I’m not an accountant, so it may be worth checking with yours to ensure this is correct. If they’ve written it off, they should be able to advise on how they did this. If you wish, you can let us know and we’ll advise on how to do this in QuickFile.