You can cancel a subevent as long as no one has registered for it.

1. On the Calendar tab, click Calendar.
2. Select the event with the subevent you want to cancel.
3. In the Actions group, click Edit Event.
4. In the Event Details window, click the Subevents tab.
5. Select the subevent you want to cancel.
6. In the Actions group, click Cancel Subevent.
7. Confirm that you want to cancel the subevent.
8. Click Save & Close.