Economic Domination

[Darth Venom]

Simple: Build more trade ports/upgrade existing ones. Torch fishing ports in provinces that can't develop any more towns.

Edit: Didn't read your post properly, sorry. If their ports are full, there's nothing you can do exept wait until they upgrade them/ build more. Checking the diplomacy screen regularly helps, to prevent AI factions from beating you to trade agreements.

On the matter of buying provinces for > 100k:
I found that this can lead to great advantages for the selling AI faction versus their AI adversaries. I bought upper Canada as Sweden from the French for ~120.000 and a few turns later they dominated the seas around Europe, completely sealing of the British Isles. Before the Brits hat complete Naval domination...