There doesn't appear to be any forum at the site you lined, so you can't be getting any spammers.
Anyhow... what you want actually covers two different scripts, one is for posts and one is for the member profile.
Rather easy to stop spammers in member profile script memcp.php by not allowing changes to the sig and bio if their post count is less than some
amount you set (like 1, which means if they have one post, you must have already approved them/unmoderated them and want them to be able to have full
access).
Just before the line:
$db->query("UPDATE ".X_PREFIX."members SET $pwtxt email='$email', site='$site', aim='$aim', location='$location', bio='$bio', sig='$sig',
showemail='$showemail', timeoffset='$timeoffset1', icq='$icq', avatar='$avatar', yahoo='$yahoo', theme='$thememem', bday='$bday',
langfile='$langfilenew', tpp='$tppnew', ppp='$pppnew', newsletter='$newsletter', timeformat='$timeformatnew', msn='$msn', dateformat='$dateformatnew',
mood='$mood', invisible='$invisible', saveogu2u='$saveogu2u', emailonu2u='$emailonu2u', useoldu2u='$useoldu2u', u2ualert=$u2ualert WHERE
username='$xmbuser'");
insert:
if ($member['postnum'] < '1') {
$member['sig'] = '';
$member['bio'] = '';
}
Untested and no guarantees!