Deactivate a user.

def users_deactivate(user):
    """Deactivate a user."""
    user_obj = _datastore.get_user(user)
    if user_obj is None:
        raise click.UsageError('ERROR: User not found.')
    if _datastore.deactivate_user(user_obj):
        click.secho('User "{0}" has been deactivated.'.format(user),
                    fg='green')
    else:
        click.secho('User "{0}" was already deactivated.'.format(user),
                    fg='yellow')