You can delete a user account without deleting the associated Contact record. This means that all of the data associated with that contact record, such as fields where they are named as the Team Lead, Monitor, etc., will remain.
When you delete the user, you will be prompted to copy their views & widgets to another user. If you ignore this step, any views and widgets created and shared by that user will be deleted.
To delete a user account, go to Settings --> Account --> Users, select the account using the check box, and click on the trash can. You will then be prompted to copy their data to another user. Select the user and choose 'OK'.
To disable a user so that they can no longer log in, you can set the user status to 'Disabled':