We received the official answer to “How Do I Permanently Remove Client Accounts From Google Adwords?” straight from Google via a support email:

“I’m happy to assist you removing client accounts from your My Client Center. First, I recommend unlinking your client accounts from your My Client Center, then replying to this email with an explicit request to cancel these accounts, including the CID and login email address associated with these accounts.

After your client accounts have been cancelled they will not accrue additional charges. While you are unable to permanently remove credit card information from your account, I am happy to completely disable the accounts so that you cannot reactivate them. I have included below instructions for unlinking your AdWords accounts from your My Client Center. If you would like the accounts permanently cancelled, please include a request with explicit language to do so in a reply to this email.”

So basically all you have to do is disassociate the accounts from your My Client Center and then call Google up to make sure they are permanently cancelled.