I have deleted a training plan, it does not appear in my calendar, but the days are still appearing in my outlook calendar. I have even disconnected the sync between Garmin and outlook but they still pop up in outlook??. yet it is only the alert about the activity you can't actually see the time in the calendar..
I haven't tried it myself, but what you're describing suggests that you'll simply have to delete the entries in Outlook independently; you can send entries from Garmin Connect to Outlook, but not automatically delete them as if it was a fully-featured calendar synchronisation.
