your observation means flexibility in jobs. I don't see why user couldn't log time again a competed job. For example, it is not rare for customers to ask for additional services after a job has been marked as completed.
in addition, this is something the approver needs to review. If approvers see the timesheet is not correct, he could reject.
I think You can try to check the Resource prices. If You can define that there is no Price for this Job for any resources, there will be an error when user tries to enter time against this Job.
In newest version of Business Central it is possible to Archive Jobs ( now called Projects).