Your first response was a good one. Move on and chalk one up to education.
As said there are 2 sides to a story, and not to be critical but if they only owe you 1500 you havent done too much , and luckily so ! Count your blessings. If you a newbie to the biz move on, maybe they are right, but if they are not go after them.
If you are really content on collecting your money start sending emails to the map.and keep sending them every week until you hit a nerve some one will get you paid but it will take a while. Good luck