Yeah looks like you have to do it with javascript but you can do it inside C2.
You can give the textbox an ID in properties and you can run javascript with the browser object.
<img src="https://dl.dropboxusercontent.com/u/8367729/construct/pics/autocompleteOff2.PNG" border="0" />
<img src="https://dl.dropboxusercontent.com/u/8367729/construct/pics/autocompleteOff1.PNG" border="0" />