Limit certain users from filling webform based on their location
You can use a module like Smart IP http://drupal.org/project/smart_ip, GeoIP http://drupal.org/project/geoip or just the Maxmind Geolite http://www.maxmind.com/app/geolitecountry table to look use the IP address of the user to determine the country.

Then use hook_nodeapi() or hook_node_view(), depending on if you are using Drupal 6 or 7, to redirect the person based on the country. 

The pseudo code would look something like this (Drupal 6, using Geoip):

function MYMODULE_custom_nodeapi(&amp;$node, $op) {
  switch ($op) {
    case 'view':
      if ($node-&gt;nid == 'NODEID OF WEBFORM') {
        $country = geoip_country_code();
        if (isset($country) &amp;&amp; $country != 'us') {
          drupal_goto('node/' . NODEID OF REDIRECT PAGE'); 
        }
      }
     break;
  }
}


edit: When I saw 'webform' I assumed you meant using the webform module, hence the nodeapi, but if you are using the basic site contact form you can use hook_menu and create a new callback for the path of the form you are using and create the same kind of check as above.