Mindtrix - put this in your phpinclude start template:
PHP Code:
$banned_words = array('sex', 'porn');
foreach($banned_words AS $bword)
{
if(strpos($bword, $_SERVER['HTTP_REFERER']))
{
echo 'Sorry you are not allowed to visit our site after visiting an adult-orientated site';
die();
}
}
Kinda silly to do it but it's up to you

P.S. it's untested, and bare in mind you can spoof referrers and a lot of browsers don't pass referral headers correctly