that cant posibly be right!
i never done a trade embargo, ill try on an old game when i get home, but i assume its just like all the other treaties. once the 20 turns is up you go to "i'd like to propose a deal", click "active" on the botom right, and then renegotiate it and cancel, that will get rid of it.
same with ROP and peace treaties
i never just go to war, i see what the other civ will give me if i renegotiate a peace with them first, i'v gotten 2 cities offered before, so if all you need is a border town that has a resource you might try it some time.
any way the "active" trades is proly the way to cancel you embargo.