The closer they are to power, the higher the cost you're going to need to make them pay. Early on you can stop them by ostracizing them, by refusing to respect them as equals, by going after their money, by chasing them out of your communities, by making revealing themselves in public a good way to get a quick beatdown. As they grow in power, the toll required to stop them increases. If you wait long enough, you'll need tank divisions; that's the history.