mass-block.js

/**
 * This is super janky, but it works. Head to:
 * https://twitter.com/followers
 * in Chrome, and bring up the web console with Command + Option + J.
 * 
 * Scroll as far down as you want the script to block (it'll block every
 * profile card that's visible on the page when you run it), scroll back
 * to the top (for the hell of it), and then paste in this script into
 * the web inspector and run it.
 * 
 * Once done, refresh the page, watch your follower count go down, 
 * and repeat as necessary!
 * 
 * I'm not responsible if this fucks something up.
 * - @cdl
 */

var cards = $('.ProfileCard');
for (var i = 0; i < cards.length, i++) {
  var card = $(cards[i]);
  card.find('button.user-dropdown').click();
  card.find('.block-text.not-blocked button').click();
  $('button.block-button')[0].click();
});

You could write a small bookmarklet.

javascript: (function() {
  $('.ProfileCard').each(function(k, v) {
    $(this).find('button.user-dropdown').click();
    $(this).find('.block-text.not-blocked button').click();
    $('button.block-button')[0].click();
  });
})();

Just stick that code into and anchor href and drag to your bookmarks bar.