[Opserty]

Just in the postbit? or are we talking about other places like the forumdisplay.php page and on the index page under the "last post" section?

If it is the former then you could try the following:

AdminCP > Styles & Templates > Style Manager > Main CSS (From the style dropdown you want to edit) > Scroll to the bottom and in the big text area at the bottom add:

a .bigusername{ text-decoration:none;}