I know your broker is not IB, but consider this:
IB makes it plain and clear in their agreement that if a trader takes a position that he is not allowed (I think meaning under-margined) then IB is responsible for any profit OR loss.
So you might want to look into this with someone who knows the law. I suspect, by law the broker is responsible for letting a trader do things that he shouldn't be able to. Anyway, if I was in your shoes, I certainly talk to a lawyer about this, and see what the dealeo is.