As of this writing, I have blocked more than 500 Twitter users, including accounts belonging to at least 20 of the 30 companies in the Dow Jones Industrial Average as well as dozens of startups. I have blocked government agencies and industry lobbies, print publications and new-media ventures, burger chains and probiotic yogurts, top-shelf spirits and sports drinks, financial institutions, automakers, cell-phone providers, over-the-counter medicines, Hollywood movies, Broadway musicals, and public television shows.