Ahh. In a later post you (AprillF) note that you are on an iPhone. That explains why in Calendar on my Mac I do not see the "i" for info that you report. I do see these things, just in other places.
In Calendars on a Mac, the Calendars are listed on the left side of the window. Just to the right of each name will be either a blank space or a "Sharing icon" indicating the calendar is shared.
Clicking on the Sharing icon opens a window with data on how that calendar is shared. If someone else shared that calendar to you, it gives that name of the person who originated the calendar. If you created the calendar, it lists addresses to which it was shared, and tells if the recipient has full access, or View Only. It also has a box which if checked allows the calendar to be a "Public Calendar" which allows anyone who has the URL read-only access, I think that if the calendar is not a Public Calendar it will not be susceptible to the spam.
The reason I think this is my Calendar has many calendars. Most are shared with specific people, and are not Public Calendars. None of these received any spam. Two of my calendars did receive the spam. One I had made a Public Calendar because it was shared with a person through a HotMail address. My guess is that access to private calendars is restricted to those with iCloud accounts, as access to shared Photos libraries is restricted. They probably are protected from invitation spam. My other calendar which received spam was created by my wife in her separate user space, and shared to me. I guessed that her original calendar was not a Public Calendar. We looked on her computer and found that it was not a Public Calendar.
I thought in might be that Public Calendars were susceptible to spam, while private calendars were not, but these facts are not clear.
Can others who received spam tell us if your spammed calendars were public or private?
