In most circumstances, if an event is cancelled after it's been advertised on the website and/or brochure, the correct way to indicate the cancellation is to follow the How to cancel a performance or an event guide. This will mark the event / performance as cancelled on the website and automatically remove the booking button.
However, if you have set an event to display on website in error and you want to remove it - this is how:
Go to your WordPress dashboard
Click on Eventotron and your festival/season tab
Under Available events filter for the event you want to remove. Hit the red Remove button:
Now go to the Events tab in the festival management dashboard in Eventotron
Click on the event ID and in Office Use flip Display on Website to NO
That's it.