Once logged into your account online, click on Conference and then Upcoming at the center right of your screen. Here you will see a list of all your future scheduled calls displayed month by month. Click Cancel under the call you wish to delete.
In the pop-up box that appears, just click on 'Cancel Conference' to confirm your choice. You and any participants will then be sent an email confirmation that your conference has been cancelled.
If this is a recurring call, you will be asked if you want to cancel just that one call or the entire series of calls.