How to fix hacked iPhone Calendar?
Email and calendars seemed to be hacked. How can I fix it
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If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account.
If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account.
The Apple support article is below.