Well, that brings up the question of how wide the block spans. Even if you block someone, I don't think you'd want to not be shown, say, an answer to your question if that person answers it. The only reason why I could see someone wanting to block another person from even answering one of their questions would be if the offender is spamming and/or annoying them by answering/commenting, in which case a ban is probably more appropriate. Or at the very least, a downvote.
With that in mind, the only application for the block option would be on the wall and in the chat room, which can be solved by disabling wall posts and just not visiting the chat room (not like it's all that active, anyway). Yeah, it's a less-than-ideal "all or nothing" situation, but until PM adds a better feature (assuming it ever happens), we at least have something.