To delete a trace start or trace end event you've set, right-click the event mark in the event area and select [Delete Event] from the context menu that is displayed.

You can also delete a trace start or trace end event on the Events panel which opens by selecting [View] menu >> [Event]. After selecting the trace start or trace end event (execution-related or access-related) displayed in the detailed information on the trace in the Events panel, click ( ) button on the toolbar to delete it. (See 2.17.5 Deleting events.)