Once logged into your account online, click on Conference and then Upcoming at the top of the screen. You will see a list of all your future calls. Click Cancel under the call you wish to delete.
Click on 'Cancel Conference' in the pop up box that appears 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.