Log in to your Checkbook.io account.
If you have only received a check and not created an account, use this link and click "Forgot password" to set a password to the account that was automatically created for you with the email that you received the check on.
Go to "Settings" on the bottom left-hand side of the page.
When in Settings, click on the "Accounts" tab.
From the list all the Bank Accounts linked to your Checkbook account, select the ones you want to delete (select using the first column)
NOTE: The Default account may not be deleted.
Once a selection is made, click "Delete"