If you have a subscription to Prestozon that you want to cancel, simply navigate to the Accounts menu by clicking on the circle icon in the top right. For each subscription you have you will see the option to "cancel account" at the bottom right of that account's section. Make sure to cancel each account that you want to close.
You will then be reverted to the free trial version of Prestozon at the next billing cycle (if you'd like to cancel immediately rather than at the end of the billing cycle, you can message firstname.lastname@example.org). If you'd like to revoke API access, follow the directions here https://prestozon.freshdesk.com/support/solutions/articles/22000180414-how-do-i-revoke-prestozon-s-api-access-