How to clear the Missed Activity Reminders popup

Apparently, Zoho CRM synced with my Google Apps Calendar and now, every time I log in to Zoho CRM, I get a popup window titled, "Missed Activity Reminders", but with reminders for appointments from last year. I've tried clearing these a screen at a time, but I've been using Google Calendar for many years and there are probably thousands of appointments. How do I clear these all at onece?