[@MissingAxis] Sorry to say it, but having a "no powergaming" rule doesn't tend to do shit and it seems that most people in my experience are "playing to win" over writing a story. People will still do it just because they want every edge they can get to show of their e-penis 9 times out of 10. If you can't tell, I don't have the greatest amount of faith in people at times.

As for what I suggest to do with, I know I am going to get shit by a good deal of people for saying this but, choose order over freedom. If someone is actively trying to get every edge they can, stamp them out and make them change. If someone is trying to use "creative freedom" as an excuse for anything that isn't remotely acceptable, make them change it. Your priorities as a GM is far different than that of a player for you also have to run the RP to begin with; if you fear having to be strict and use and iron fist on occasion over a bundle of roses all the time, I suggest that you harden your resolve or try something else.

A random bit: Maybe have the RP start in some kind of conflict with players already fighting a war they can't stay out of. It's give the RP some basic initial plot direction and let people vent their warmongering out. That said, you'd need faith in people to be able to accept that they aren't going to win every battle and be okay with "loosing" unless you want to either intervene yourself or use random numbers/dice rolls.