Katerine, you should make changes in this file: /ow_system_plugins/admin/views/controllers/users_index.html
Simply comment the following lines:
<div class="ow_box ow_admin_search_box ow_smallmargin">
{text key='admin+search_by'}
<select name="search_by">
{foreach from=$searchQ item='label' key='question'}
<option value="{$question}"{if $currentSearch.question==$question} selected="selected"{/if}>{$label}</option>
{/foreach}
</select> :
<input type="text" name="search" id="username-search-input" style="width: 280px" value="{$currentSearch.value}" />
{decorator name='button' type="submit" langLabel='admin+go'}
</div>
P.S. Do not forget to enable DEV and DEBUG modes:
http://docs.oxwall.org/plugin-tuts:enable-debug?s[]=debug