in the blocks.html file for that theme
Code:
<table width="170" border="0" cellspacing="0" cellpadding="5">
<tr>
<td>
<table width="170" cellpadding="0" cellspacing="1" border="0" class="forumline">
<tr>
<th width="170" colspan="2" height="22" nowrap="nowrap"><font class="block-title">$title</font></th>
</tr>
<tr>
<td class="row1" width="100%"><font class="content">$content</font>
</tr>
</table>
<table border="0" cellpadding="0" cellspacing="0" class="tbl">
<tr>
<td class="tbll">[img]themes/iCGstation/forums/images/spacer.gif[/img]</td>
<td width="100%" class="tblbot">[img]themes/iCGstation/forums/images/spacer.gif[/img]</td>
<td class="tblr">[img]themes/iCGstation/forums/images/spacer.gif[/img]</td>
</tr></table>
</td>
</tr>
</table>
remove the br tag at the end. You can also decrease the cellpadding in the first <table tag, that will decrease the dead space around the block