Cayd12 wrote:
Did you find a solution to stopping unwanted invitations in calendars? I have same problem
I do suspect this is coming from a Microsoft mail service. They have had that trouble for a very long time and the calendar invite is coming directly from their servers that are synced with your Calendar app. Along with the links provided by MrHoffman, there are many more on the Microsoft Support site.
You can stop syncing that email account with your Calendar and the problem will be solved, otherwise you will need to work through the suggestions Microsoft provides so they don't appear in your account. The issue they have with the spam affecting their servers comes in waves where they can control it for a while, then it pops up again and adds the spam events to your calendar automatically.