It seems Twitter has strong armed Dossy's Twitter Karma and Huitter's Mutuality service out of providing the bulk unfollow function on Twitter. It was very useful for those trying to grow their Twitter following. You could bulk unfollow those who were not following you and fill those available slots with new followings hoping that some of them would follow you in return. This follow – wait a few days – bulk unfollow and then rinse and repeat worked very well for growing your Twitter following.
Well Twitter didn't like that and seems to have succeeded in blocking these two popular services from bulk unfollowing. What to do now? Unfollowing one person at a time is way too much work and you'll soon give up. Our preference is using services like TweetSpinner. They have a free and a paid version. Even the free version allows you to unfollow 20 people at a time with one keystroke. That's a lot better than manually unfollowing one at a time.
Their paid version is very smooth. You set your parameters and they unfollow in an automated fashion on a regularly scheduled basis. Once you set it, you don't have to be involved any more. It does it automatically.
TweetSpinner does not mass unfollow / bulk unfollow in one stroke but does so a few at a time all day long. This keeps Twitter at bay and also helps you grow your account. In addition to the bulk unfollow function, TweetSpinner also provides filtered following, smart tweets, scheduled tweets, rotating profiles and automated DirectMessages. Well worth the nominal monthly fee. Use my link and get a free one week trial.
Another service that can help is JustUnfollow. It presents you a list of those that don't follow you. You can unfollow them, one at a time. You cannot unfollow 20 at a time or 10 at a time. Only one at a time. But they are presented separately so you don't have to sift through.