Do a mod_rewrite in you .htaccess file.
It's not going to really be helpful to cut/paste code since there are four different versions of the code that is acceptable available, but there is a code generator and testing section available at
http://www.htmlbasix.com/disablehotlinking.shtml
I used it to generate the code for my site. Found out I was using more bandwidth in serving the graphic smacking hotlinkers than they were stealing. Just turned it all off.
Made this though and it seems to have made the rounds.
