There a two options there, with different outcomes.
Deactivating a team member
If someone is leaving, and you want to keep her/his history on TeamMood, that's the way to go.
Deleting a team member
Use this option if you want to delete the team member and all her/his whole history of data (moods and comments).
WARNING: we cannot undo the deletion, so please make sure you know what you do.