If your the account owner (the one who created the account), you can delete your account and cancel your contract at any time. If you do so, you will only pay for that day but no longer.
In order to do so, login to your CentralPlanner account, go to the Account settings in the top right corner and click the Destroy account link.
We're sorry to see you go, but wish you all the best. Thanks for giving CentralPlanner a try!
related article: contract term