Guests can be deleted from an Event altogether from the full-screen view of Attendee Manager.
First, select each guest using the checkboxes to the left of their name.
Next, click the red Delete button in the lower-left of the screen.
Finally, double-check the guests to be deleted, and click OK.
To quickly delete all guests from an Event, first select all guests using the upper-most checkbox, in the column header row...
...and then select the red Delete button.