Until now, when you blocked people on Twitter, they couldn't follow you and wouldn't see your tweets in their streams. Of course, harassers could always go to your public profile and read your updates there, but it would take a little extra effort on their part.
However, Twitter has changed its blocking policy, so that blocking a user really just mutes them from your point of view. A blocked harasser can now follow you, read your tweets in their stream, and even retweet them. The only upside, as far as I can tell, is that blocked users will no longer be able to tell that you've blocked them.
Anyways, before Twitter made this latest change, it was super-easy to make someone unfollow you without permanently blocking them, if you so desired. All you had to do is block them and then unblock them.
With the new blocking policy, this just involves a couple more steps:
- Select the option to "Protect my Tweets" under "Security and privacy" settings.
- Go to the profile of your unwanted follower and block them. This will make them unfollow you.
- If you want, you can then unblock them (or not).
- Just make sure to unprotect your tweets to make them public again.
This won't stop someone from re-following you, but with any luck it'll be a while (or never) before they notice that they "accidentally" unfollowed you.