How to remove calendar spam?

That is a Rogue Calendar account. 

If you are using iOS 13 then Go to Settings ➔ Passwords & Accounts and under Accounts see if an unknown calendar account is listed. If it is then tap on it and then Delete account.

If you are using iOS 14 the Go to Settings ➔ Calendar ➔ Accounts and under Accounts see if an unknown calendar account is listed. If it is then tap on it and then Delete account.

Note that it is not a virus. It is something that got added to your calendar because you may have accepted an offer from a pop-up on a website unknowingly. Please be careful out there.

Axel F

Calendar spam you signed up to get.

You need to remove the spam, as you downloaded it, maybe restore the phone