Unwanted ads in your calendar come from an spam calendar import or subscription. By far the most common way is the calendar subscription method.
The technology is the same as WebCal.Guru uses, but spammers use it for evil instead of good. These calendar subscriptions can't appear automatically, but the user must accept them. But if you do not expect a calendar subscription confirmation, it is easy to accept it unintentionally.
Fortunately it is quite easy to get rid of the spam calendar.
Look for the calendar you wish to unsubscribe and tap on the icon beside it.
Important! Make sure you delete the right calendar. Usually it is named to something inconspicuous, such as Events. You can find out the calendar by clicking open one spam calendar event.