Prevent Microsoft 365 meeting invites from automatically appearing in users’ calendars

That option "automatically process meeting requests..." as far as I know is unchecked by default and also does not work to counter meeting request spam. The meeting will still be added to your calendar as "tenative" and this is "by design" from MS. User posted his settings, showing that setting as unchecked and still gets meeting/calendar spam, also as soon as you select the message or view it in the reading pane it will be added to yoru calendar as tenative despite that setting. An Exchange Transport rule might be your only option.

Reference:

https://learn.microsoft.com/en-us/answers/questions/4729168/outlook-automatically-accepting-scam-meeting-invit

https://kb.uwm.edu/uwmhd/45596

https://learn.microsoft.com/en-us/answers/questions/5558001/how-to-prevent-calendar-invites-from-external-sour

In New Outlook or Outlook on the web, go to the Calendar view, then on the Home or View toolbar, click the Filter button. (The button will say Filter applied if the calendar is already being filtered.) Mouse over the chevron next to Meetings, then uncheck Not responded.

I don't believe there is any way to enforce this setting org-wide, unfortunately. In my experience, New Outlook doesn't even retain the user's selections here as I always want to see my Reservations but Outlook keeps hiding them.

May refer this:

https://optiable.com/how-to-stop-spam-calendar-invites-from-automatically-appearing-in-outlook/