Is there a module/solution for allowing users to block each other on an SNS site?
Take a look at the Ignore User module.  From the project page:


  This module allows a user to "ignore" content from other users that
  they do not wish to view. Node and comment content from an ignored
  user is hidden from view on a page. A message is displayed above the
  hidden block of content with a link to the user's "ignore list" and a
  link to toggle display of the ignored user's block of content in that
  node or comment.


Not sure if it will handle blocking a profile view or not, but it should block content.