게시판의 테마가 basic이라면 

skin폴더의 board폴더의 basic폴더로 들어가서 list.skin.php에 있는 내용중에

       <option value="mb_id,1"<?php echo get_selected($sfl, 'mb_id,1'); ?>>회원아이디</option>

       <option value="mb_id,0"<?php echo get_selected($sfl, 'mb_id,0'); ?>>회원아이디(코)</option>

       <option value="wr_name,1"<?php echo get_selected($sfl, 'wr_name,1'); ?>>글쓴이</option>

       <option value="wr_name,0"<?php echo get_selected($sfl, 'wr_name,0'); ?>>글쓴이(코)</option>

 이부분을 지워주면됩니다. 

경로는 그누보드루트폴더>skin>board>해당게시판테마>list.skin.php입니다.

<!-- 게시판 검색 시작 { -->

<fieldset id="bo_sch">

    <legend>게시물 검색</legend>

    <form name="fsearch" method="get">

    <input type="hidden" name="bo_table" value="<?php echo $bo_table ?>">

    <input type="hidden" name="sca" value="<?php echo $sca ?>">

    <input type="hidden" name="sop" value="and">

    <label for="sfl" class="sound_only">검색대상</label>

    <select name="sfl" id="sfl">

        <option value="wr_subject"<?php echo get_selected($sfl, 'wr_subject', true); ?>>제목</option>

        <option value="wr_content"<?php echo get_selected($sfl, 'wr_content'); ?>>내용</option>

        <option value="wr_subject||wr_content"<?php echo get_selected($sfl, 'wr_subject||wr_content'); ?>>제목+내용</option>

        <option value="mb_id,1"<?php echo get_selected($sfl, 'mb_id,1'); ?>>회원아이디</option>

        <option value="mb_id,0"<?php echo get_selected($sfl, 'mb_id,0'); ?>>회원아이디(코)</option>

        <option value="wr_name,1"<?php echo get_selected($sfl, 'wr_name,1'); ?>>글쓴이</option>

        <option value="wr_name,0"<?php echo get_selected($sfl, 'wr_name,0'); ?>>글쓴이(코)</option>

    </select>

    <label for="stx" class="sound_only">검색어<strong class="sound_only"> 필수</strong></label>

    <input type="text" name="stx" value="<?php echo stripslashes($stx) ?>" required id="stx" class="frm_input required" size="15" maxlength="20">

    <input type="submit" value="검색" class="btn_submit">

    </form>

</fieldset>

<!-- } 게시판 검색 끝 -->