John R. Ellis: Technically, you can't. But you can block them.
John Skeats: Having people add you to their circles is arguably harmless. If a person adds you to one of their circles, that only allows them to see posts that you identified as Public or posts you shared with Extended Circles if they are in the circles of one of the people you circled. They cannot see posts or anything else that you share with your circles (unless, of course, you add them to your circles). It is not like friends on Facebook, which is a two-way relationship.