Viruses

You got tricked into installing a rogue calendar; your phone does not have a virus.

If your phone has iOS 13 or earlier: Go to Settings/Passwords & Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.

If your phone has iOS 14: Go to Settings/Calendar/Accounts, Tap Subscribed Calendars, tap on the offending calendar and delete it.

And in the future beware of "free" offers

You’re welcome! There are a lot of scams like this around. Here is some useful information from Apple→Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support