Hmm... yes, it wouldn't be a major hack to not create new users, but I'd like to offer the intended alternative. There's nothing preventing you from keeping user accounts invisible to customers. If you change the messages and adjust the e-mail, you can still get the advantage of associating orders with a user account from the back end without ever revealing the accounts to the users. Simply set it to create the new users as blocked, don't send new account e-mails, and make sure no notifications are being sent out with login details or links.