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.