As far as I can remember, to prevent hotlink in Drupal is not that easy. If you search website to look for prevent hotlink, you will find a lot of them to use with .htaccess file. But those are not for Drupal. You can't just use it in Drupal .htaccess file, that won't work. However, I have a set of code that work great. Actually, I have this code long time ago but I can't remember where I got it from. Thanks for the guy that provide it. Too bad that I didn't save link at that time.
So, now, here is what we have to do :