in that cause (im basing this from the fisubsilver theme) simply open up header.htm in ure theme folder and look for

Code:

<td width="150" nowrap><font class="content">$theuser</font></td>

near line 13, simply delete

Code:

<font class="content">$theuser</font>

to remove it.

the php for it is in the theme.php file but u can leave that alone if u want.
if u still have any trouble just say so and ill find out if u need 2 delete the php 2.