Once logged into your account online, click on Conference at the top of your account home page and then 'Upcoming' at the center right of the screen. You will then see a list of all your future reservations organized by month. Just click the 'Cancel' button under the conference you wish to delete.
You and any participants will then be sent an email confirmation that your conference has been canceled. If this is a recurring conference, you will be asked if you want to cancel just that one call or the entire series.