My phone is infected

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

Your phone is not infected. You authorized some dicey website to install a spam calendar.

Be careful out there! Don’t accept any offers from websites or apps.

See the post from Stedman1, above.