Set the default timezone you want everyone to have via the Admin Panel > Settings, then disable the opportunity for users to change that:
Remove or comment out the following:
include/staff/mypref.inc.php
<tr>
<td nowrap>Preferred Timezone:</td>
<td>
<select name="timezone_offset">
<?
$gmoffset = date("Z") / 3600; //Server's offset.
$currentoffset = ($rep==NULL)?$cfg->getTZOffset():$rep;
echo"<option value=\"$gmoffset\">Server Time (GMT $gmoffset)</option>"; //Default if all fails.
$timezones= db_query('SELECT offset,timezone FROM '.TIMEZONE_TABLE);
while (list($offset,$tz) = db_fetch_row($timezones)){
$selected = ($currentoffset==$offset) ?'SELECTED':'';
$tag=($offset)?"GMT $offset ($tz)":" GMT ($tz)"; ?>
<option value="<?=$offset?>"<?=$selected?>><?=$tag?></option>
<?}?>
</select>
</td>
</tr>