We have an app that monitors resource mailboxes, and applies validation checks when one accepts a meeting invite from a user. If these validation checks fail, the code makes the Graph API call to tell the mailbox to decline the invite.
We are encountering a scenario where the Graph call to decline the invite fails. Specifically it's when the user sent the invite with "request responses" unchecked. The Graph call fails with the message Your request can't be completed. The meeting organizer hasn't requested a response.
So basically the users have found a way to work around our validation system, by simply unchecking "request responses" when inviting these resource mailboxes to meetings.
Edited to clarify my question: Is there any way, via the API, to tell a resource mailbox to decline in invitation, if the organizer unchecked "request responses?
We worked with MS on this and figured out why it was doing it. It turns out, if the user sent the meeting invite with "request responses" unchecked, you can't send them a response, either through the API or user interface (Outlook). So basically to decline the invite via the API (Graph), you have to include this in the request body:
"sendResponse": false
Unfortunately it means the user won't receive any indication that it was declined, but at least you can do this and the call won't throw an error.