Copy the relevant template from themes/sample/theme.php into your own theme.php and comment out the button using html commenting.
// HTML template for the image navigation bar
$template_img_navbar = <<<EOT
<tr>
<td align="center" valign="middle" class="navmenu" width="48px"><a name="top_display_media"></a>
<a href="{THUMB_TGT}" class="navmenu_pic" title="{THUMB_TITLE}"><img src="{LOCATION}images/thumbnails.gif" align="middle" border="0px" alt="{THUMB_TITLE}" /></a>
</td>
<!-- <td align="center" valign="middle" class="navmenu" width="48px">
<a href="javascript:;" class="navmenu_pic" onclick="blocking('picinfo','yes', 'block'); return false;" title="{PIC_INFO_TITLE}"><img src="{LOCATION}images/info.gif" border="0px" align="middle" alt="{PIC_INFO_TITLE}" /></a>
</td> -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{SLIDESHOW_TGT}" class="navmenu_pic" title="{SLIDESHOW_TITLE}"><img src="{LOCATION}images/slideshow.gif" border="0px" align="middle" alt="{SLIDESHOW_TITLE}" /></a>
</td>
<td align="center" valign="middle" class="navmenu" width="100%">
{PIC_POS}
</td>
<!-- BEGIN report_file_button -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{REPORT_TGT}" class="navmenu_pic" title="{REPORT_TITLE}"><img src="{LOCATION}images/report.gif" border="0" align="middle" alt="{REPORT_TITLE}" /></a>
</td>
<!-- END report_file_button -->
<!-- BEGIN ecard_button -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{ECARD_TGT}" class="navmenu_pic" title="{ECARD_TITLE}"><img src="{LOCATION}images/ecard.gif" border="0px" align="middle" alt="{ECARD_TITLE}" /></a>
</td>
<!-- END ecard_button -->
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{PREV_TGT}" class="navmenu_pic" title="{PREV_TITLE}"><img src="{LOCATION}images/prev.gif" border="0px" align="middle" alt="{PREV_TITLE}" /></a>
</td>
<td align="center" valign="middle" class="navmenu" width="48px">
<a href="{NEXT_TGT}" class="navmenu_pic" title="{NEXT_TITLE}"><img src="{LOCATION}images/next.gif" border="0px" align="middle" alt="{NEXT_TITLE}" /></a>
</td>
</tr>
EOT;