Seriously, everytime I ask my buddy nations to stop trading with my enemy they always turn back to trade with them after a turn or so. It also happens with the Breaking Alliances function, which is particularly annoying to France, which she must destroy every Coalition nation in her path.
I think that Trade Embargoes and Break Alliances should have a time limit like Military Access with 5, 10, 20 turns..etc and Indefinite, which requires the other nation to keep their promise for a long time instead of just one turn. And you can still break it, of course (all treaties are made to be break ), historically it happens to Portugal and Russia, but you will suffer a big diplomatic penalty.