Well first of all, Welcome to TechIMO...
Secondly if its a toolbar that loads with Internet Explorer there should me a setup screen, or setting area. I know there is for my Yahoo toolbar, but i use FireFox, in there you should see settings to be able to disable remembering your search history and/or prediction or predicting what you want to ask.
It might even be called AutoComplete, if you go to tools>options and then go to the content tab and click on AutoComplete that might solve your problem, but that works all throughout IE not just for the Yahoo Toolbar, which might be separate. So i would check that plugin's (yahoo) own settings.