The Hunger Games

You've completely mixed up the concepts here. "Ignorance of the law is no excuse" is irrelevant here. The point is that it's impossible to prove that the second player, who ends up winning, knew about their ally's violations. They can simply say, "I don't know anything about it," and claim that the second player acted independently and wasn't warned. Furthermore, in the format of this tournament, alliances are conditional, and proving their existence is also problematic – Player A might say they have an alliance with Player B, while Player B will say they know nothing about any alliances, that no one offered them anything, and that they didn't agree to anything.
The only thing that can help is the logs of the correspondence, but they can be deleted on both sides, leaving no trace, or a recording of the game where these correspondences are visible, but few people show their chat during the broadcast.
Speculations about Player A having to know everything that Player B does cannot be the basis for an accusation; facts are needed.