You could try something like this:
RewriteEngine on
RewriteCond %{HTTP_REFERER} ^http(s)?://(www\.)?domain1.com.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http(s)?://(www\.)?domain2.com.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http(s)?://(www\.)?domain3.com.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http(s)?://(www\.)?domain4.com.*$ [NC]
RewriteRule .* http://www.google.com [R,L]
Does a great job on my sites

Btw, I got this from:
http://www.htaccesstools.com/block-hitbots/