How to cancel trade routes in Freeciv

I had established several trade routes with another player when both of us agreed to a peace treaty. However, the deteriorating political situation has forced both of us to cancel the treaty and declare war on each other.

Now I want to place a trade embargo on that player, and cancel the trade routes to that player's cities because the trade routes are helping the enemy reach technology and financial goals (both of which I find unacceptable). I can comfortably survive without those problematic trade routes, but I've determined that the enemy cannot survive without them.

But how can I cancel trade routes that go to enemy cities?

I know that I can try to find better trade routes in hopes of replacing the problematic trade routes, but …

1. The replacement of old trade routes is not guaranteed as new trade routes have to yield more trade points in order to replace old trade routes.
2. The seas are now infested with enemy battleships, cruisers, submarines and aircraft carriers, making it extremely impractical to establish long-distance trade routes that can replace the problematic trade routes.