in JC settings there is a setting that will prevent any transactions to be posted to a closed Job

If the operator has access to Setup Jobs set in Security Administration, the operator then has the ability to change the status of the job.  There is not a setting that will prevent an operator from changing the status if the operator has access to Setup, Jobs. Recommendation would be to limit who all is able to access Setup jobs, or alert all users with this ability that the Job status is not be changed and who they should contact if entries were generated to a closed job.

Limit who can set up jobs by removing that role from them.  Then if they want to post something against a closed job they have to come to that individual and request them to open the job back up.  You can't limit them from changing job status is they have permission to setup a job in project managment or job cost.