The easiest way to do this is to comment out the relevant <tr> tags to take out the whole row. The smarty way to comment is by using {* at the beggining of the block to remove and *} at the end.
eg:
if you want to get rid of the fax no:
Code:
{*
<tr valign=middle>
<td class=FormButton>{$lng.lbl_fax}</td>
<td></td>
<td nowrap>
<input type=text name=fax size=32 maxlength=128 value="{$userinfo.fax}"></td>
</tr>
*}