Such a mass banning of the nodes would effectively be creating a private network, would it not? (Although coordination levels would probably have to be 100% to prevent leaking between networks through a node)
Which is impossible. Valid transactions on one network will ALWAYS end up reaching the miners on the other network if these transactions are also valid there. We saw this with ETC/ETH. Miners on the other network have all incentive to process these transactions to collect the fees in *their* coin. In the end, they go and READ THE OTHER public BLOCK CHAIN to get the transactions on their chain.