Well, you can always revert to code obfuscation. In the end, Flash is a client side run software, so eventually the received data must be readable by the player, meaning it is also readable by anyone else.
The only way around that would be a deeper integration of such features in the Flash Player itself. Which is, afaik possible in combination with the Flash Media Server. But I'm not sure whether it's DRM options extend to more than just video.