You could use the [Relation module][1]. Create a many-to-many user-to-user relationship, call it "block_user" and allow users to add relations.

Whether you use relation or flag (see heshanlk's answer): If you do not want to write code, you could replace the default comments with a view in a block. See [How to display comments of current node in a view][2], and just add another filter for the relation or flag. You will be filtering with two contexts: the current node, AND the current user.


  [1]: http://drupal.org/project/relation
  [2]: http://drupal.stackexchange.com/questions/80525/how-to-display-comments-of-current-node-in-a-view