You can't show them something on your site if you are telling them that they can not see your site. Either send them to another domain, dialer page or whatever.
Try this: (This will handle the domain / all subdomains / and all TLDs for "some-referer" ;) .
Code:
RewriteCond %{HTTP_REFERER} ^(http://)?(www\.)?.*(-|.)some-referer(-|.).*$ [NC]
RewriteRule ^(.*)$ http://www.forum-leeches-here.com [R,L]