Previously I wrote an article, located here, about the NK2 file on Windows that stores the email addresses you type into the “To Field” when sending emails. The NK2 file stores the history of what you type or paste into the To field so it can auto complete for you in the future. This is a great feature however deleting old entries is not always the easiest and that makes it a bit unfortunate. You can typically turn off auto complete however you could also modify the files that store the contact cache that is used for auto complete. Follow the directions below to locate email addresses stored in the To Field Outlook for Mac cache to either modify or delete.
I am sad to say that currently there is not a legit way to turn off Auto Complete on the iPod Touch. Many people might recommend to Jailbreak the Touch so you can modify anything including the auto complete functionality. In my opinion Jailbreaking your iPod Touch should only be done if you have way above average technical skills so this would rule out most people.
There are a lot of people who claim all you need to do is SSH or FTP to your Touch or iPhone and you can modify a file and turn off auto complete. You can only SSH or FTP to your phone if it is jailbroken. Again I would not recommend doing this unless you either have a spare Touch/iPhone or your technical ability is above average.