You have to be either the Event Organiser (owner of the HeySummit subscription) or a Team Member with permission to delete the event. Team Member's permissions can be edited by the Event Organiser under My Account > Team.
- Log in to the event and select Overview > Manage (In the top right hand corner, with the drop down menu)
- Here you will find a red Delete option.
- Click on Delete and select Yes in the pop-up window.