With "Proof of Stake" to mean a consensus system, are you saying the Ethereum Proof of Stake implementation doesn't do anything to guarantee against a cartel of bad actors?
My understanding is "slashing" may be one such mechanism. This is where staked funds are taken away from bad actors. ie if you propose a block with bad transactions, the 32 ETH you have staked will be reduced by some fraction as punishment. When your stake is slashed down to 16 ETH you are booted from the system. So that us a clear punishment against a bad actor, though I'm not sure what a cartel means i this case. Perhaps you know about this already however.