Ok, I have a bit of a weird problem. I hate the Celts, they are very far away, but insist on being at war with me, they are one of the two largest empires, of fairly equal size (the other being mine, Roman). I'm very annoyed, it's a pain to fight off their waves of cavalry, when I have other, closer enemies. So I signed an embargo against them with every nation except three, Carthage, who is trading with them, Scandinavia, who is also trading with them, and the Celts themselves all refuse to sign an embargo. But I REALLY want them to suffer,

it's nice that the non-trading nations aren't going to start trading, but I want Celts to be unhappy and deprived of whatever resources aren't in their empire.
I can't stop Scandinavia from trading with them, but Carthage consists of a large peninsula, locked off by me and only me. There is also one city elsewhere on the map, no matter. I want to blockade Carthage, I would destroy them, but I have other fish to fry, Carthage is wither still in the AA, or barely out of it, they are one of, if not the least powerful/advanced nations.
Can I blockade them with privateers? That is my only viable plan, also, ho do you blockade? I have never done it before.