You're never going to be able to stop alliances from dominating an island. It's what happens to every alliance that's successful for long enough. People don't like breaking up a winning team.
I agree with Tom that the only solution to this is to kill them. I however don't think that it is realistic considering that infiltrators are weak and would need their chances turned up by around 5000% before they could actually successfully be a drive of political change on continents. Rulers are rich, they have big units. It's already near impossible to stab a guy with a unit and get away with it, let stand one with a BIG unit. And if you try and you fail, you get banned. You won't try again unless you want to get executed and if you get executed you can go and spend another year in the academy before you'll be able to try again.
This is the reason why bounties don't or barely work and it's the reason why there's no such thing as infiltrator guilds anymore. Infils are simply too weak.
