How do we stop closing a job if a PO is still active on that job?
Let me rephrase this:
How do we stop a user from being able to close off a Job in NAV if ledger entries are still active on this job?
Currently it is possible to close the job off without an error message.
I'm not sure what you mean "PO is still active". Is it received and non-invoiced items or quantity on PO not received?
If you mean on second topic, system don't know what quantity you want to receive, but if you mean on first topic, it can be a small problem because you can not post invoice for this quantity if Job is in status Completed. But you can change status again to Order and post invoice. This is only way to do it.
If you need some automatic control, you have to make customization in situation when you change Job status to Completed, but I think it is not necessary.