I thing that have a virus

Open Calendar App from the Home screen.

Next, tap the Calendars button at the bottom.

You should now notice a section called On My iPhone with several suspicious calendars listed below it. ...

Tap on the i button next to the suspicious calendar.

Now, tap on Delete Calendar.

Are you seeing something as shown in the pic below? This is neither a hack nor a virus. It is spam and you can get rid of it. Read below...

Then probably you have accepted an offer or subscribed for a spam Calendar accidentally. Watch the GIF below to know how to unsubscribe from these alerts. Look for an unknown Calendar on the device under subscribed calendars.

Open your calendar app and tap Calendar at the bottom, if there is a calendar you do NOT know, tap it and delete it. You can also go to Settings -> Calendar -> Accounts. If there is an account you do not know, delete it immediately.

It's not a virus. On some websites there are pop ups where there are invitations to participate in a kind of "SPAM calendar" you have clicked add without thinking about it. I have tried it myself and you just delete it again.