Virus

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 or higher, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

The Apple support article is below. 

Remove calendar spam

Video support

You probably don't actually have a virus. What happened is a malicious website probably added a bunch of events to your calendar to make you think you have a virus. In your calendar app click on "calendars" at the bottom in the middle. If there is a calendar you don't recognize click the "i" symbol next to it and then scroll down and click "Delete Calendar." If it made the events in your main calendar just make a new calendar and delete your current calendar. You will lose your current calendar, but at least you won't have constant pop-ups.

Did you try the YouTube video? If you go back to wherever you agreed to this rogue calendar it will come back.