You can delete supplier invoices yourself. If you want to do this, the invoice must be in "Draft" status.
If a supplier invoice has "Paid" status, you must first reset the status to "Open" by deleting the payment under "Outgoing payments". More information on how to reprocess supplier invoices can be found here: Edit bills
"Open" or "Overdue" status
If the status is "Open" or "Overdue", you can click on the three-dot menus on the right of the status to reset the status of the invoice to "Draft".
"Bank payment entered / downloaded / transmitted" status
Delete the bank payment under "Payments" and the status of the supplier invoice will be reset to "Open" or "Overdue".
Now you can use the three-dot menu on the right of the status to delete the invoice permanently and completely.