the following code center the scroll and hide scroll bars:
var scrollBarWidth:int = 15;
var maxHScroll:int = html.htmlLoader.contentWidth-html.width+scrollBarWidth;
var maxVScroll:int = html.htmlLoader.contentHeight-html.height+scrollBarWidth;
html.horizontalScrollPosition = (maxHScroll /2);
html.verticalScrollPosition = ( maxVScroll /2);
html.verticalScrollPolicy = "off";
html.horizontalScrollPolicy = "off";
see this link for more info on how to handle HTML dom events in air.