From what I know about Steemit and blockchains I think it is basically impossible to properly implement a block. Everything you've posted is public information on the blockchain and can be read by others. The implementation of "mute" in the Steemit web app is just a convenience for users. They could implement a block as a convenience but it would only work on the Steemit website and could be easily thwarted by someone using multiple identities and other ways to browse the Steem blockchain.
Someone, please correct me if I'm wrong in that.
That said, I understand your desire to have this - it is sad that so many folks need to hide behind anonymity these days to protect themselves from dangerous and deranged stalkers who can't control themselves in public (or private I'm sure).
Most of the people harassing others are too stupid to continue harassing them if they are unable to do it through the steemit gui. I do think block would be a great thing. Mute is nice, but there are also annoying people that you'd prefer not to be able to see your posts. It would also help alleviate the situation where someone decides to be a dick and flag all of someone's posts. If the person was blocked they would not be able to see someone's posts on the Steemit GUI and flag them.
Good point about flaggers. I will say that I know a fair number of people who use multiple identities to make Steemit posts (either to try and attain some anonymity, or to cheat the up-voting system). On the mobile apps it is relatively trivial to flip between them.
At the moment Steemit has a relatively small user community in the few hundred thousand range. Although I think proportionately we might have a few more "fringe" users than elsewhere we don't seem to be anywhere near the absolute huge numbers of "dicks" that tens of millions if not hundreds of millions of users would bring.
Too many dicks - that pretty much sums up many problems in life :-(