The Embargo Option (in the foriegn relations tab)?

It will shut down all trade routes between you and them and stop any from being created until you un-check the box.

I'd be interested in stories about when and why you might ever use an embargo. I've never done it either because it never seemed useful. Maybe its a way to antagonize someone who had multilple trade routes to your empire into attacking you so you don't have to be the agressor?

I've done it to try and choke every last penny from someone I'm trying to undermine w/o my military.

Like, if I'd prefer the Dregin to quit their war with the Arceans, but I didn't feel like bribing them for a 1 time peace deal, I'd just pull the plug from the trade routes to them and try to sell any 'useless' tech to them to drain their coffers there.  Then I'd drown them in spies.  The more money I can keep from hitting their factories, the less ships they make, the shorter their war.

Moreso, I use it as a 'cold shoulder' towards someone I don't really like.