First of all like Xenosapien said, If you don't want to be betrayed, don't ally.
Contrary to most, I don't really have a problem with "breaking" of alliances. They are not blood pacts. In reality alliances shifted all the time, because in reality there is no such thing as an ally win. Factions ally to gain advantage or for a mutual interest but eventually these interests can change. For example USA and USSR were allied to destroy the Nazis but once they defeated the mutual they no longer had a common enemy and broke the alliance. Or in medieval japan clans would be allied to defeat the dominant clan but then break alliance once of their allies got too strong. An atwar example, lets say you are allied to this player who is just getting huge and powerful and destroying the remaining players but you could shift the balance and gain most of his lands if you end the alliance. Why do people settle for second place? Are you a beta? Changing alliances can makes the game really interesting.
Anyway I avoid ffa games altogether. Team games or locked 0 alliances please.