That is a judges agreement, which I think works fine in the game.
We also made a strict rule about spies, both for & against us - be interesting if anyone genuinely would go along with that.
Is it possible to have a "clean" island? 
Not really, just like in RL no matter how many rules you impose, there will be some who attempt to operate outside of them. Certainly you should be able to make them a very small group though.
