What would then prevent a small number of players with a large (near majority?) number of coins going from miner to miner requesting a "buy-in" to prevent them

What would then prevent a small number of players with a large (near majority?) number of coins going from miner to miner requesting a "buy-in" to prevent them from being blacklisted? It seems like this sort of behavior (unless totally abhorred by the community) could very quickly poison it.