Problem:
user points out that in the invoicing process he sets invoices draft to paid after payment. As a result, the linked job is set to CLOSED.
In some cases he reopens the job again, and by procedure, approves it again. It appears that invoices are set back to INVOICE DRAFT status again and therefore might be invoiced by the supplier for a second time.
Is there any settings which locks INVOICE PAID status and makes it impossible to return to INVOICE DRAFT status again?
Solution:
In order to block the invoice status go to:
Settings / Invoicing/ Invoicing Processes 
and enable the option: Block if Invoice Paid