Step 1: Terminate the Provisioned Product that launched, created or enrolled the member account, from the Service Catalog Console. This process is also known as unmanaging an account from Control Tower.
When you terminate an Account Factory account in AWS Service Catalog, the account is not closed. This action removes the account from its OU and your landing zone.
https://docs.aws.amazon.com/controltower/latest/userguide/account-factory.html?icmpid=docs_ctower_console#unmanage-account
Step 2: Remove the member account from the AWS Organization.
In order to remove the account from the organization, it needs to be able to operate as a standalone one. Hence, it is necessary to complete the sign-up steps.
https://docs.aws.amazon.com/organizations/latest/userguide/orgs_manage_accounts_remove.html
Step 3: Login to the AWS member account with Root credentials and close it
To close an account, you must be signed in as the AWS account’s root user.
https://aws.amazon.com/premiumsupport/knowledge-center/close-aws-account/