Adding a person to your block list makes your profile (and all of your other content) unviewable to them. Any connections you have to the blocked person will be canceled. To add someone to your block list, visit that person's profile page.
Who can view your profile?
Who can post on your profile?
Which of the following things do you want to have published about you in the recent activity feed? Note that changing this setting will only affect future news feed items.
