Yes. To do this you must go to "Mail Options" and in the Personalisation category choose "General Preferences". At the very bottom of the page you will see an AutoComplete setting under Message Actions.

Here you will see two options:

1. "Use Address AutoComplete (default)".
2. "Do not use Address AutoComplete".

You can change these settings at any time.

Note: when you turn off Yahoo!Xtra Address AutoComplete, it will affect every computer you use Yahoo!Xtra Address Mail on, not just the computer you are currently using.