We can have 2 types of debts from clients that we might want to delete.
1. Delete debts without receipt or invoice associated.
When there is a debt in the customer's file but no invoice or receipt has been generated yet.
Another way of knowing where the debt comes from is going to Debts tab, inside the client's file, on the Billing and collection section.
Inside this debts tab, we can access the service directly and cancel it to delete the debt. To do it press 
and it will take you to the service. For example, we click on a booking and it will take us to the booking screen. To delete this client's debt we will need to delete him from the booking. In the case it's a debt caused by a school, once you press 
you'll see a screen with all the fees of the service and you can just delete the ones outstanding pressing the red cross or the document with a green zero, if it's a subscription fee it will take you to the subscription file and you'll need to unsubscribe the service on a date before the last outstanding fee, so the debt will be erased, and so on with each debt.
2. Delete debts invoiced but outstanding.
Ticket or invoice has already been generated but payment is still pending
The process would be the same, but if you see on the debts screen that the debt has a document number on the Cash receipt/invoice column, then you can't delete the debt, you'd have to do a credit note to cancel the receipt or invoice. You can see how to do that on the next link: