"Colluding: Players working together is not against the rules, and you should not report players for doing so. To avoid collusion, play in 1 v 1 games where collusion is impossible, or in team games (since then the collusion is pre-arranged), or just play with friends you trust."
Players working together is not against the rules. I'm sure in a real life Risk game you've sat at a game where someone has said "X is the strongest player, if we don't attack X we lose." It's smart in a free for all for players that encounter each other early and have alternative methods of expansion to not attack each other.
There's an entire community in the game called "Diplo players" in which the game revolves around playing by pre-arranged rules of alliance building. I think this is probably the sanest way to play free for alls.
The real sanest solution is don't play free for alls as inherently they will put your fate into the hands of the other players.