Use ActionScript. You know... the build-in scripting language for Flash...
I'm not much of a ActionScript programmer, so I couldn't tell you off the top of my head how to do it. I'm pretty sureh though, you could check the referrer from the movie using ActionScript either in a onLoad event or some other event for when the movie is downloading. If it's not correct then stop downloading, or don't allow the movie to be played.
... hope that helps. You should check out some of the ActionScipt boards for any help you need on ActionScript. here's a good one:
www.actionscript.org/
cya
-Adam 🙂