You could make war on them both and then make them both your protectorates. But that would switch the balance of power rather much, so if you want a 'cheat-free' game, you might not find this to your liking...

You could also force other factions to attack the one you wanted to be stopped - but this might in the long run endanger this one, or even strengthen it even more, if it managed to win these wars also ;)

Generally, the ability to make two AIs make peace with one another is SORELY MISSING in ALL COMPUTER GAMES. Game developers dont consider that it has some strategic value that your ally doesnt expand too much so they think we dont want to be able to influence how our friends behave...