There are 2 ways you can Delete a Event that is part of a Series. You can delete just the one event or the whole Series of Events.

To Delete a event(s) navigate to:
  1. onMessage
  2. Content
  3. Events
  4. Select the Event Category the Event is listed in

If the Event is listed without the recurring event icon (arrows in a circle) the recurring event is not selected therefore if you click the red X to delete this will only delete the one event.User-added image

If it were part of a recurring event series which is designated based off the recurring event icon (arrows in a circle) being highlighted in yellow User-added image

when you click the red X you would get the option of:

This event is part of a series. Would you like to apply this to the selected event or the entire series? 

Apply to selected event
Apply to entire series
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If you select the Apply to selected event this will only delete the event you clicked the X from. 

If you cselect the Apply to entire series this will delete all of the events within the series.