Unfortunately, by design, Ethereum is not a permissioned (i.e. [Eris][1]) block chain (anybody can interact with the block chain given they know at least a bootstrap node's address).

The answer to [this][2] question overviews the access control mechanism used by Eris. 


  [1]: https://ethereum.stackexchange.com/questions/1012/what-is-the-difference-between-ethereum-and-eris
  [2]: https://ethereum.stackexchange.com/questions/4568/what-property-makes-proof-of-stake-easier-to-apply-in-permissioned-blockchains