My calendar has been hacked

You got tricked into installing a rogue calendar; your phone does not have a virus.

If your phone has iOS 13 or earlier: Go to Settings/Passwords & Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.

If your phone has iOS 14: Go to Settings/Calendar/Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.

And in the future beware of "free” offers.

Note that while you can alternatively uncheck it in the Calendar app, that does not actually remove the calendar, it just hides it.

The easiest fix is:

1. Open Calendar
2. Tap Calendars, at the bottom
3. Uncheck anything that look inappropriate