I had the same problem and finally managed to figure it out. Under My Portfolio, go to My Accounts -> Account summary page. Click on the account you want to delete. When it expands out, click on the Edit tab. You will see a stack of little horizontal bars towards the end of the little account panel (much like the same you see when entering a comment here). Click on that and the entire panel should expand. Now if you scroll to the end of the panel, there are Update and Delete buttons for the account.
Hope that helps.
