The video won't start loading until you set the source property, so just set it with code when you want it to start
sims281's answer works as long as you don't want to use that FLVPlayback again later, since his method will work, but will mess up the player. I made a class a while back that does stop the video stream effectively and still allows you to continue using that player though, you can see it here:
FLVPlaybackUtils
Using that class you can just do this:
import FLVPlaybackUtils;
FLVPlaybackUtils.reset(myFlvPlayback);
It will clear the player, stop any streaming or downloading, and reset the player to be usable again, just in case you want to use it again later in the program.