Then what you are asking for is impossible. At some point, coders make choices on what they implement that has an impact on the protocol and the network. Furthermore, it is practically impossible to pretend that you can prevent people from arbitrarily choosing to point their clients to another blockchain. Forks will happen because of bugs, developer intervention, and even because a user has decided to make his own personal fork for whatever reason. There's always a game going on about how many people are going to join a given fork.

The only truely "decentralised" system is the universe. You can not roll back anything, time passes and the laws of physics take care of executing themselves with no possible changes on the "blockchain". No coin that is decentralised in that sense can exist. Cryptocoins are simulations of a universe, but humans are one level on top of it, and the protocol can not reach out and prevent somebody from fiddling with the controls. It would be like sort of Skynet world playing God with its own creators.