In certain cases you might run into the issue of having a bunch of validated (test) invoices in your database even before going live with accounting. In order to clean those out, follow the next simple steps: 

1. Create a server action and create the contextual action for it:

for am in records:  

  am.button_draft()

  am.write({'name': '/'})

2. Go to the invoice model and make sure all invoices you want to delete are in 'cancelled' state

3. Select said invoices > action > execute server action 

4. Finally, don't forget to delete the server action once your invoices have been deleted. Indeed, this kind of action is considered as a python code execution, and if you are not on your own repository, it will generate code maintenance fees to pay every month.