On a blog, however, no such authority exists

Well, blogs do have administrators, who hold a similar authority. I believe Eliezer has banned several people from LW for making only poor quality or trollish posts, for instance.

anyone can make such requests - but only in response to breaking certain social norms without a good excuse.

Well, yes, anyone can make such requests, just like I can request that LW commentors refrain from using the word "the" because I find it incredibly offensive. The point is that it isn't a reasonable request. If someone's violated enough of the community norms to be banned, that's a matter for the administrator, but that's different than an individual requesting "please don't reply to my comments in a public discussion forum" as if it were comparable to "please don't email or call me."