My main goal is to have a Project file that will contain old and new employees (inactive and active). If an old employee finished an old task he/she was assigned to, I want him/her to stay assigned to that task but unable to show up as a valid resource when assigning to new tasks.
I ran into this question where it's explained that using the active/inactive column is only available on MS Project Server. We are using MS Project Desktop. I tried creating active and inactive groups as suggested in the linked question, but I can't find how to assign special rules to those 2 different groups.
I also tried using resource availability, but that doesn't disable the resource from being assigned to a task. The resource can be assigned to a task, but if the units are at 0% the work won't be calculated. That won't work for my case.