User is Inactive but still show up in reports or as available resources?
Making a user inactive after they have left the company will not stop a user from showing up in reports because the Active field only controls whether a user can log into Project Insight.
The second role to consider disabling is "Project Resource" under System Roles because this controls when adding resource to a project whether the inactive user still displays.
Then the last item to consider once a user is Inactive and no longer a Project Resource, that it does not remove the user from any existing assignments, meaning if the user was on a project and assigned tasks they will continue to show up on any reports that have to do with those projects and on those projects they are still resources they can still be added to tasks as resources. The user must be removed from the project in order to stop showing up as an option when assigning resources to tasks and/or various reports.