If you need to cancel a meal, simply go to the page where you added a date for your meal. Then click the "change" button next to the date and select "cancel this date". The meal will be removed from search, and any guest that is signed up will get a full refund.
If you have guests that are signed up, it is always good practice to shoot them a message letting them. Also, you can always add new dates when you are good and ready :)