Depending on your level of familiarity with Flash, it'd probably be easiest to load up the FLA in Macromedia Flash, then just make a new layer, then put a Keyframe on the last frame of the animation, and under actions put in
stop ();
That should halt the playback, but it also depends on how they constructed the FLA to begin with, you may need to dig into some MovieClips as well. Just make sure you save a backup before doing too many changes.