Rule #2: No secret alliances
Any form of collusion between opponents must be announced beforehand in the game chat, in English or in a language all players understand. Secret alliances can be hard to prove, but if you suspect one you should leave the players in question negative feedback. If the players have a history of suspected secret alliances consider reporting it following the instructions at the top of the Cheating & Abuse Reports forum.
Got a question about this one, especially about the 'announced beforehand'. How is that defined? At the start of the game? Within the first 'n'-rounds? Or doesnt it matter when it is announced in the game chat?