Like I said, I think its entirely up to the player to move the units from that location. Other wise is just a case of use it or lose it! However, that relies on them being honest about what's supposed to be there. "Oh no that was just a bluff! I never intended anything to Ambush from there!"I often remove barbarian APs during the game (not in the scouting phase). What would you do then?
It's just a childish game of "I've thought of a number".
Well that's an interesting suggestion in itself but really its not solely about Ambush for me. Although, the fact that you can currently bring your entire force out of a high standing terrain AP, while the Romans just stand there and look at you is pretty ridiculous.Perhaps restricting Ambush to fanatics, cavalry and skirmish troops might be an alternative solution for you.
Its far more about the ability for unlimited Deployment from any point designated and the fact the Barbarians can just plonk down anything they like, well over half way across the table, and pretend that it was their intent that these Groups were there from the outset.