What Facebook does is to discourage people from adding people they don't know in the first place; evidently this hasn't worked. One interesting thing they used to do but don't anymore: back when they kept track of where people knew each other from, the dialog box for adding someone as a friend would ask where you knew them from, with a whole bunch of checkboxes. One was "I don't know them"; checking this would, rather than displaying boxes so you could enter more details, instead display the message "Then why are you adding them?", and change the "add friend" button to a cancel button. (I may have the exact details wrong there.)