I had the same problem today. 50 identical emails were blasted to all calendar subscribers. The emails continued every 2 minutes until I deleted the calendar entry. The email was supposed to be sent 1 day prior to the activity. The emails began exactly 24 hours before the activity ... but continued for almost 2 hours until I could get to a computer to delete the event.
On another activity the emails are supposed to be sent out 2 days prior to the activity, but instead they are delivered 15 minutes prior to the activity.
There appears to be an issue with the email reminder feature.