Ensure that you followed exactly the steps in this article: Set up multiple calendars on iPhone – Apple Support (AU)
"You can unsubscribe from a calendar you’re subscribed to. When you unsubscribe, you can also report the calendar as junk.
- Tap Calendars at the bottom of the screen.
- Tap
next to the calendar you want to unsubscribe from. - Tap Unsubscribe, then Tap Unsubscribe or Unsubscribe and Report Junk.
- Note: When you tap Unsubscribe and Report Junk, the calendar is reported to Apple as a suspected junk subscription."
If the above steps still doesn't work or if you had accidentally subscribed to a spam calendar, try the steps in this article: Delete spam calendars and events on iPhone - Apple Support