That's my solution as well.
Civ V has a dynamic that, if you're playing for domination, after you've captured two or three capitals, you can pretty much count on three or four civs to simultaneously declare war on you. I find this a challenge to my play, because my military is often stretched pretty thin / overcommitted to one front. But more importantly, I find it entirely plausible. If one country has shown such aggressive propensities, the others are going to be more inclined to band together to stop that one. So I find this a natural anti-snowballing effect.
It think it could be enhanced. If you've conquered two capitals, any civs that share a border with you should sign a mutual defense pact and should move maybe 50% of their troops to those borders, prioritize the building of citadels there. Late game, with air and naval power, it shouldn't even be just civs that share a border with you; it should be the rest of the known world. And they should set their troops up for some focused, maximally-annoying counter-defense, so that even a civ that hasn't built a particularly large army can set you back a step.
And, there should be some equivalent in other dimensions of the game. They're more inclined to sign research agreements with one another, if you start to develop a tech lead. They boycott your blue jeans and rock-and-roll if you get too uppity in culture.
And the player should be able to join in such leagues to keep a snowballing AI in check.
And "hear, hear" to GeneralZift's x-post.