Validation on creation vs editing of a termination

Summary:

We have a rule which checks if the line manager is terminating an employee. He should not be able to terminate an employee, only a contractor, that part of the rule is working. However, when an employee resigns, the line manager needs to edit the termination before submitting it. How can we build in that the line manager cannot create the termination of an employee but only edit it? Is WhenWhy.CreatedBy an option?

Many thanks in advance for your help