Spam calendar appointment can't be declined, how to get rid of it?

I have a month-long meeting from a spam account appearing in my calendar app. It is not a spam calendar, just a spam event. There is no evidence of the appointment on the email account the calendar is associated to, I cannot find it on any other version of this same calendar, just the one on my phone. I've seen the advice to delete the spam calendars or unsubscribe for them, which is not applicable in this situation. I decline the appointment, it vanishes for a second, then comes back immediately. I can't block the sender. I can't delete the appointment. Please help me make it go away!