In a BTS game (Noble level) I'm looking at a situation where my only opponent on my continent is getting his badly-needed horses via a resource trade with a civilization on another continent reachable by galley (via coastal squares; nobody should have Astronomy yet at 200 BC). Is there any way for me, a 3rd party, to stop this particular resource trade?

There are a variety of circumstances where I might want to (or have wanted to, earlier in the game): Now, when I'm at war: would placing a galley smack in the middle of each coastal route block a resource trade?
Earlier, before war: would such galleys have blocked routes before we opened borders?
There appears to be a critically-placed island where I could conceivably build a city whose borders expand to cover the appropriate coastal squares. Without open borders, would that block the resource trade?

1)If you activate blockade mode for the galley(s), then yes, assuming the blockaded region covers all possible trade routes between the two civs.

2)No, since I don't think you can blockade outside the waters of a civ you're at war with.

3)Yes

I believe Galleys are unable to blockade, but Triremes (req: MC) can blockade.

Thanks for the clues. I finally thought to look at the orders for galleys, which include the cannon icon for "blockade": "The unit disrupts the trade route so that no intercontinental trade can pass through the blockaded tile." So, presuming "trade route" covers the resource trades, the galley can do it but since one galley blocks one tile I might need many -- so the city option might work best in my particular circumstances.

Note that blockade does cover a large square (something like 7x7) around the blocking ship, but only the sea and ocean squares. You can see the area blocked (by all your ships) by selecting a ship having blockade orders (unfortunately you do not see it when giving the order to blockade, so you should first select a different unit and then reselect the unit you just gave the blockade order to see it).

Thanks -- I plopped in a couple of galleys with the World Builder to check things out, and selecting a single galley showed the blockade region for all of them.

A more detailed question:

If I have my island city occupying the only coastal route between continent 1 and continent2, and I want to prevent all resource trades between X (my enemy) and Y (some other guy I might or might not want to stay friendly with):

If I close borders to both, it seems from the above answers and the results I'm getting in my game that the resource trade gets prevented. At least, I'm much farther along in this restart as I was in the old game where they'd started resource trading by now.
If I open borders with Y (the non-enemy) can the resource trade happen, or must borders be open to both X and Y for them to trade resources?

I am actually unsure here. I would think that if you opened borders to Y, the resources he bought would arrive, but the resources X bought would not. At least, that's what I think :sad: