Deactivates the specified MFA device and removes it from
        association with the user.

        :type user_name: string
        :param user_name: The username of the user
        
        :type serial_number: string
        :param seriasl_number: The serial number which uniquely identifies
                               the MFA device.

def deactivate_mfa_device(self, user_name, serial_number):
        """
        Deactivates the specified MFA device and removes it from
        association with the user.

        :type user_name: string
        :param user_name: The username of the user
        
        :type serial_number: string
        :param seriasl_number: The serial number which uniquely identifies
                               the MFA device.

        """
        params = {'UserName' : user_name,
                  'SerialNumber' : serial_number}
        return self.get_response('DeactivateMFADevice', params)