This short guide is for those who want to invade a neighbour whom they have a Trade Agreement.
Guide to Break a Trade Agreement
Unfortunally, there is no way to break a trade agreement once established unless you decide to go to war, as far as I remember.
If you want to invade another country that you have a trade agreement with and don’t want to suffer the penalties (your merchants won’t be happy), send a spy over to “ruin relations” with you. They’ll eventually declare war with you.
Other than that, don’t make agreements with countries you plan to invade.