I would definitely blockade their harbors if I could get away with it, but in the event it may not work (because of peace), I would try to take measures with the country (s) who might be trading with them.
If you are still at war with them, I would either take out their harbor or the city, or at least destroy all roads joining their harbor to the rest of the country.
Wish I had a definite yes or no answer. I've got to check this out myself.
EDIT - Normally in peace time, naval blockaids do not work, but there may be exceptions due to circumstances.