To Delete an Event and its' Runbooks, a selection of Runbooks in the Event or just the Event itself and leave the Runbooks intact click the Details icon from the Event Summary panel.

Click the Delete Event button in the top right hand corner of the Event Details panel

In order to delete an Event and all Runbooks within the Event, you must be an Event Manager and a Deployment Manager of all the Runbooks within the Event. If the user deleting the Event is not the Deployment Manager of all the Runbooks they will only be able to delete the Runbooks for which they are a Deployment Manager as shown above.

Select which Runbooks you wish to Delete along with the Event and click on the Delete button.

You will be presented with a Deletion Confirmed message.