The simplest way to do it, although you may want to explore more to remove the outside border, is to find the begin_container() function in reserve.template.php.
Just comment out (or remove) the following lines:
<td class="tab-not-selected" id="tab_advanced" onclick="javacript: clickTab(this, 'pnl_advanced');" style="border-left-width:0px;"><a href="javascript:void(0);"><?php echo translate('Participants')?></a></td>
and
<td class="tab-not-selected" id="tab_additional" onclick="javacript: clickTab(this, 'pnl_additional');" style="border-left-width:0px;"><a href="javascript:void(0);"><?php echo translate('Accessories')?></a></td>
That will remove the tabs for those two in the popup.
As I mentioned, you'll still have the "basic" tab, but you can take that out in the same function. I suggest you experiment with it a bit,
by commenting out pieces. If you erase, you could break it pretty ugly.
rob