It's a little difficult without having the raw template to work with, but in your showthread template just before <!-- time zone and post buttons -->, try deleting the following code:
<!-- spacer -->
<table bgcolor="#FFFFFF" width="100%" cellpadding="0" cellspacing="0" border="0">
<tr><td width="10"><img width="10" height="1" src="images/space.gif" alt=""></td><td width="100%">
<!-- spacer -->
|