To stop all hotlinkers use this code in your htaccess file (stored on your root directory):
To allow certain domains to hotlink, just copy the two RewriteCond lines and add in the domains you want to hotlink.
Oh, one other quick point, htaccess is a bit fussy depending on your server's apache setup and mod rewrite options, so its always best to consult with your hosting company if you have issues getting it working.
cheers,
Luke