Wodan
Deity
Ships can already effect a blockade.
Simply stationing a ship outside an enemy city will prevent that city from working the tile the ship is on, plus all 8 adjacent tiles. This reduces both food and commerce for the city.
Oh, I realize you're probably talking about temporarily halting actual Trade Routes of the city. But, in reality that's small change to the amount of commerce you are actually costing the city by preventing the coast tiles from being worked.
Wodan
Simply stationing a ship outside an enemy city will prevent that city from working the tile the ship is on, plus all 8 adjacent tiles. This reduces both food and commerce for the city.
Oh, I realize you're probably talking about temporarily halting actual Trade Routes of the city. But, in reality that's small change to the amount of commerce you are actually costing the city by preventing the coast tiles from being worked.
Wodan