Because the keystroke function is built into the engine, and not scripted (I'm pretty sure that ambience is just an action, and not a condition) we have no way to disable it without altering the source.
If I'm wrong and it is a condition as well (ambiencegreater=?) then you can create a global script, attached to a light or something, that watches the ambience and puts it back to 0 when it changes. Alternatively, creating multiple zones along the players path that change the ambience to 0 may work, that is if the script can override the engine.
Best.
I'm sorry, my answers are limited. You must ask the right question.