Rothbard said that we should reject out of hand any inconceivable state of affairs as a goal for human beings and that any move toward such a goal should also be rejected.
http://library.mises.org/books/Murray%20N%20Rothbard/Egalitarianism%20as%20a%20Revolt%20Against%20Nature,%20and%20Other%20Essays.pdf
And just so there’s no doubt about Rothbard’s view that communism is an inconceivable goal for human beings, here is Rothbard on Marx:
http://mises.org/daily/3769
