Removing a subscribed calendar from an iPhone
Where a subscribed calendar can be deleted depends on how it got there. Work through these in order: unsubscribe from inside the Calendar app, delete the subscription account in Settings, check for an installed profile, and finally check whether the calendar is actually synced from a Google or Outlook account rather than subscribed on the phone.
Work through these, most likely first
1Unsubscribe inside the Calendar app
Calendar app > Calendars (bottom) > tap the info button next to the calendar > scroll down > Unsubscribe or Delete Calendar. Most feed subscriptions end here.
2Delete the subscription in Settings
Settings > Apps > Calendar > Accounts (older iOS: Settings > Calendar > Accounts). Feed subscriptions appear as 'Subscribed Calendars'; tap in and delete the one you're done with.
3It came from an installed profile
Calendars added by schools or employers sometimes arrive in a configuration profile. Settings > General > VPN & Device Management: removing the profile removes its calendars. You can't delete just the calendar while the profile stays.
4It's synced from Google or Outlook, not subscribed
If the calendar lives in your Google account, deleting it on the phone is impossible; untick it in the Google Calendar sync settings or remove it in Google Calendar itself, and the phone follows.
Rarer, but worth knowing
It comes back after deletion. That's the sign it is account-synced (cause 4) or profile-installed (cause 3): the source keeps re-adding it until you remove it at the source.