How to remove calendar spam?

1. Open the Calendar app.
2. At the bottom of the screen, tap Calendars.
3. Look for a calendar that you don't recognize. Tap the More Info button  next to that calendar, then scroll down and tap Delete Calendar.

If this doesn't fix the issue, delete the calendar subscription in Settings:

1. Open the Settings app.
2. Tap Calendar > Accounts. Or if you use iOS 13, tap Passwords & Accounts > Accounts instead.
3. Tap Subscribed Calendars.
4. Look for a calendar that you don't recognize. Tap it, then tap Delete Account.

Settings>Calendar>Accounts... delete the rogue calendar you (even accidentally) subscribed to.

Regards

Giulio