Correct, there is no way to do it, and I consider it a serious weakness. I should be able to request that two civs stop fighting, if in no other way than offering them a reward for doing so. Also, as a powerful nation, you should be able to say: hey you, stop your war against X or I will attack you.
This bugged me a lot in my last game on Diety, where I had a mutual protection pact with France. Germany attacked me and the French dutifully came in to help me out and got completely trampled in the process. I set out to altruistically liberate France from the Germans, but in the middle of my doing it I got attacked by someone else who had mutual protection signed with France. So right as I'm wiping out the last Germans from their territory, after I gave the French 5 liberated cities (3 with wonders) as presents, they declare war on me. I refused to hurt a single Frenchman, but they refused my constant pleas for peace for centuries. I eventually got pissed off at them and decided to win back and keep the cities I gave them. Their stupidity made the game much less enjoyable for me.
I really wish alliances were more flexible.