To close your agency account, navigate to the Agency Settings page, and click the link that says "Delete agency account".
You will then be prompted to confirm that you want to delete the agencies account, as doing so will remove all records for users, clients, projects, messages, files etc associated with that agency account (unless a user is also tied to another Outlign account in some way).
Once you have confirmed the deletion process you will be logged out and all data will be removed.