How do I delete events in my iPhone calendar posted by Apple
You can disable unwanted calendars in Calendar.app on iPhone with these steps:


open Calendar.app in your iPhone
Hold your iPhone vertical
On the bottom are three buttons visible: Today, Calendars, Inbox if you don't see them, go back to the monthly overview)
Tap "Calendars" > Hide All Calendars
Now select only the calendars you want to see.
Tap "Done" when you're done.


Let me know if this works for you.