From there, you will need to select Followers, which is at the top of the page, above the pictures of people in circles.
On their profile page, look for the little tiny dot-dot-dot next to the Follow Box. When you click on that, it opens a menu that allows you to block this profile.
Remember that followers aren't the same as friends, so just because someone's following you, it doesn't mean that you have to follow them back, and see their stuff. Google+ isn't like Facebook that way. But some followers are just spam, and it's good to make them go away.
I suppose Block Profile is so deeply hidden so that people don't accidentally click on it. I really have no idea. But if someone is following you who is obviously spam, or pornography, go ahead and block them.