very very simple:
edit template "postbit" or "postbit_legacy" depending on the one you use.
FIND:
HTML Code:
<div id="postmenu_$post[postid]">
<if condition="$show['profile']">
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]">$post[musername]</a>
$post[onlinestatus]
<script type="text/javascript"> vbmenu_register("postmenu_$post[postid]", true); </script>
<else />
$post[musername]
</if>
</div>
... simply delete that part! ... it's the line where the username is displayed.