If the appointment is organized by Alice (organizer) and send to Bob (attendee) you should decline as Bob (attendee):
var decline = invite.Decline("bob@example.com");
This creates an iCalendar entity with Bob's participation status set to Declined:
BEGIN:VCALENDAR
PRODID:-//Google Inc//Google Calendar 70.9054//EN
VERSION:2.0
METHOD:REPLY
BEGIN:VEVENT
DTSTART;VALUE=DATE:20191230
DTEND;VALUE=DATE:20191231
DTSTAMP:20100508T173704Z
ORGANIZER;CN=Alice:mailto:alice@example.com
UID:e595e21e-2d0c-4966-b127-4fd40233d7eb
ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=DECLINED;CN=
Bob:mailto:bob@example.com
STATUS:CONFIRMED
END:VEVENT
END:VCALENDAR