To cancel your plan, please downgrade the site(s) to Basic which is free forever! You can do this at any time from the Plans & Billing page by using the Data Collection Rate drop-downs.
This will cancel your recurring plan so that you will not be charged again. You will still retain the plans you paid for till the end of the current billing cycle.
Please note that only Account Owners can access the Plans & Billing page and / or alter plans.