This question interest me mostly in the context of the Ethereum, but it might be interesting to see how this problem is solved by the other cryptos as well.
I understand that the consensus about what is included in the ledger of the next block is established by the game-theoretic disadvantage of the miners who mine a block that includes ledger entries which are likely to be disputed by majority of the other miners.
But as a mining pools get bigger and sometimes advocate using their own software to mine the coins, once their hash rate exceeds 50% they will have a power to form their own consensus, based on the whim of the admins of the mining software.
With that power they will be able to block certain transactions to get into the chain, effectively freezing assets on any accounts they don't like and disturbing the execution of the smart contracts.
This situation can happen even more easily, when the ETH would switch to the proof-of-stake, since it is easier to accumulate a capital rather than to accumulate the hardware.
What are (if any) provisions in ETH and other crypto currencies against such scenario?
I find it is strange and somewhat disturbing, that such a fundamental question did not got answered. AFAIK the situation I described is very real concern for the bitcoin: https://www.cs.cornell.edu/%7Eie53/publications/btcProcFC.pdf . How does the Ethereum fare?