When an employee no longer works for your organisation or at least the part that ROSTERkey is being used to manage, you should take the following steps to remove their access.
Firstly, you should disable their login. This is done by an Administrator or the Configuration -> System -> User page.
Secondly, you should remove them from any Crew that they were in. Use the Operator Movement - simple mode to set their "New Crew" to "None" as at the date on which they are no longer part of the organisation.
Thirdly, you should terminate their active period on that same date. Use this process, Setting Workers as Active or In-active, to set the correct "To" date for their current active period.