sample .htaccess This one is opt-in abusers. most times it's used the other way round.
SetEnvIfNoCase Referer ".*evilcorp.*" block=1
Order Deny, Allow Deny from env=block ErrorDocument 403 youstealimages.png
<FilesMatch ".(gif|jpg|png)$"> Order Deny,Allow Deny from env=block </FilesMatch>
ciao, tom