Everyone,
following above instructions to remove the calendar from iOS calendar app did not work for us - with iOS 11 the delete option does not show up. However, one would be able to deselect the calendar for visually showing up in the native calendar app. Be aware that due to some iOS bug users with an Apple Watch seem to get still reminders for meetings set up in the calendars that they chose to not show in the calendar app (that is, getting reminders on the Watch, as well as new calendar invites on both Watch and iPhone).
The only practical way to remove another persons calendar from your mobile (if synchronized via Exchange ActiveSync) in our case was to let users log in to OWA, then have them remove the additional calender from the OWA calendar view. This would not remove the calendar from Outlook clients on PC or Mac, but remove it from OWA and ActiveSync devices. Just deselecting the "Show calendar" option in native iOS calendar app was not the way to go for us due to the iOS bug.
It would be great if these side effects could be mentioned in the calendar access invitation emails that get sent from O365 (like, "if you click on the button to accept the access invitation be informed that this will add the persons calendar on your mobile phone") or just name the button "add calendar to all my devices".
Issues around this behavior were difficult to analyze due to the fact that only peoples calendars were affected that recently had been granted access to, or changed access rights, as well as mostly manager calendars being affected. People and our service desk would typically have checked the access rights on the shared calendar itself instead of the reporting users calendar list in OWA (in our organization, most people would not even know about OWA). Hey Microsoft, there's room for improvement here.
