More often than not, you might want to disallow or ban someone from following you on Twitter.

When you ban or block someone, they won't be able to add you to their "following" list. You can't add them to your "following" list -- I know you don't intend to. Further, they won't be able to send messages to you. They will still be able to see your activity and messages on public view of Twitter, but that's it.

Here's how to ban someone from following you on Twitter:

If the person is already following you or if you are following that person, go to the "Follower" or "following" list, find the person you want to block, and hover your cursor on the "blue" button that says "following" or "follower" respectively. That's when the button turns into red and says "unfollow". Click on it.
Find the person's profile page and "block" the person and you're done.
Special social media tools such as Social Bro, Nimble, etc, can also help you do the same, if you are using those tools, that is.