You could use the Relation module. 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 viewHow to display comments of current node in a view, and just add another filter for the relation or flag. You will be filtering with two contexts: the current node, AND the current user.