Stolen info alerts

You've likely been offered and then subscribed to a spammer's calendar from some website.

Tap the Calendar button at the bottom of the Calendar app, scroll down to the Subscribed Calendars, find the calendar that matches the color of the alerts, tap the information button off to the right, scroll down, and select delete to delete the subscribed calendar.

More info > Delete spam calendars and events on iPhone - Apple Support

If these alerts are via your calendar, see:

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. 

https://support.apple.com/en-us/HT211076