easy-block.user.js

// ==UserScript==
// @name           twitter Easy block
// @namespace      http://z3c.info/
// @description    Adds the block button on user's profile page
// @include        http://twitter.com/*
// @exclude        http://twitter.com/*/*
// ==/UserScript==

$ = unsafeWindow.jQuery;

(function () {
  if (!$("#follow_control")) return;

  $("#follow_control")
    .append($("<a/>")
      .attr("href", 
        "http://twitter.com/blocks/confirm/" +
          $(".follow-actions").attr("id").replace("follow_actions_", ""))
      .append("block")
      .css("color", "grey")
      .css("vertical-align", "-20%")
      .css("padding-left", "1ex")
    );  
});