You don't appear to show any attempt to apply the format to the textfields. You either set it as the default before you add text or set it to the format after it has been written to the textfield. Here is a link to help documentation that will explain...
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/fla sh/text/TextFormat.html
As far as nulling goes, there is no attempt shown for that as well, so it is not really clear (to me) what you intend/mean as far as nulling it. To clear a textfield you can just assign iut an empty string... as in ""