We automatically pull in accounts if we see their account ID and line items within a consolidated Detailed Billing Report. Removing a linked account will only be fully removed if it has no associated costs in the last 13 months.
We suggest changing the account name to include a naming convention like 'Deleted', so that these accounts are easy to identify. Be advised that the account name can only be updated when the API section of the account edit page is filled in. In other words, you won't be able to rename it if the account has been deleted from AWS.