A: Mark the appointment as "Private" in your calendar. The Office 365 Calendar and Google Calendar apps never sync private or confidential events to Upsales. The event still appears in your Upsales calendar marked "Private", so others can see that you are busy, but no appointment is created in Upsales.
Marking an event as private after it has synced does not remove it from Upsales. Delete the appointment in Upsales. This does not delete the event in your calendar.
Only meetings with at least one attendee outside your company's email domains are synced. An appointment without guests, or with colleagues only, is not synced even if it is not private.
With Google Calendar you can also turn on Do not sync events with default visibility in your app settings. Then only events marked as public in Google Calendar are synced.