Google should be fixing this on their end, but they haven’t yet. And they have no way to report spam (see below for an update on reporting spam through the web interface). If you’re seeing something like the screenshot below, read on for how to fix it (via The Verge, among others).

I’ve seen a few Google calendar spam invites popping up automatically. If you’re seeing these, too, or want to prevent them, here’s how to fix it:
- In Google Calendar, click the settings icon.
- Navigate to Event Settings.
- Select No, only show events to which I have responded.
- Navigate to Events from Gmail.
- Uncheck Automatically add events from Gmail to my calendar.
- If you’re like me, and have multiple Google accounts and/or calendars, repeat steps 1-5 for each calendar.
- Share this article with friends and family – the links in these calendar invites are not friendly and you don’t want anyone you know clicking on them.
Good luck!
*Update 08-27-2019*: In the web interface of Google Calendar, you can click/tap the vertical ellipses and “report as spam” to report the event and remove it from your calendar. (via Diane L. Smith)

As far as I know, this is not currently an option in the mobile Google Calendar app.