If you want to use the new option and disable trade post buildings in your current game you will have to edit your save file. You will be able to do that only if your save file is not compressed. You can disable compression ingame.

1. Open your save file in text editor.
2. Look for "game rules = "
( Below that line inbetween symbols { } are listed all game options )
3. Wherever inbetween those symbols add new line that says exactly as follows:

MEDIEVAL_TRADE_ROUTES="MTR_BUILDINGS_OFF"

4. Save changes and enjoy your game with no trade post buildings.