They were telling them that they had to funnel all their trade through New York. To do otherwise would have cost them enormous fines and penalties, up to and including the loss of the entire cargo and any ship carrying it.
Which law passed by Congress and signed by the President authorized those actions?
Navigation act of 1817.
It created prohibitive penalties for using foreign ships or crew to carry this trade.
Once the South left, they could have hired foreign ships and crews to do the shipping and it would have been a great savings compared to what the Northeastern monopoly shipping cartel would have charged.