If you don't have server access, then one thing you can do is alter your mailing script to ignore requests from that address. Are you using PHP to mail the form to you? if you are, then you could do something like this:
PHP Code:
if ($_SERVER['remote_addr'] == 'ip.add.here')
{
//echo message about using being banned
}
else
{
//usual mailer script goes here.
}
Not sure about CGI ones, as I don't use Perl.
HTH.