If you go to Panel > Billing & Account > Manage Account you will see the hosting Service Plan for your account. At the end of the information there's a (tiny) Edit link which you can click and set it to "End Today".
If the account is new you have a 2 week trial period during which you can cancel hosting. Cancellation can be initiated via Panel and/or Support Ticket. If you're concerned that they might not "get on top of it" before the trial period ends, simply remove any CC or other method of payment immediately so that you can't be charged anyway.