You need to be editing this template:
skin1/help/contactus.tpl
Comment out or remove the table that shows the Departments:
Around line #237
Code:
<TR valign="middle">
<TD class="FormButton">{$lng.lbl_department}</TD>
<TD><FONT class="Star">*</FONT></TD>
<TD nowrap>
<SELECT id="department" name="department">
<OPTION value="All" selected>{$lng.lbl_all}</OPTION>
<OPTION value="Partners">{$lng.lbl_partners}</OPTION>
<OPTION value="Marketing / publicity">{$lng.lbl_marketing_publicity}</OPTION>
<OPTION value="Webdesign">{$lng.lbl_web_design}</OPTION>
<OPTION value="Sales">{$lng.lbl_sales}</OPTION>
</SELECT>
</TD>
</TR>