Although I have never actually tried this, you may consider using ammonia based window cleaner (windex) and soaking the model in it. After anywhere from a few hours to the next day, the paint should start to lift off and should be easy to remove with basic tools. (i.e: paper towels, hobby knife in some harder to reach places.) I personally have actually had quite a bit of luck with airbrush thinner. Although more expensive, it works instantly. Do not soak the model in it though...If used in excess, it can warp or dry the plastic so you have to use more caution. Perhaps some other folks can add to this.... Hope it works out for you!
-Rich