RewriteEngine on
RewriteCond %{HTTP_REFERER} ^http(s)?://(.+\.)?badsite\.com [NC,OR]
RewriteCond %{HTTP_REFERER} ^http(s)?://(.+\.)?badsite2\.com [NC,OR]
RewriteRule \.(jpg|jpeg|png|gif)$ - [NC,F,L]
# If you want to display a "blocked" banner in place of the hotlinked image,
# replace the above rule with:
# RewriteRule \.(jpg|jpeg|png|gif|bmp) http://example.com/blocked.png [R,L]