It seems that you cannot unfriend someone once he or she has friended you, even if the "friendship" is involuntary and unwanted. Now I have something in common with my teenagers!
I recently had someone friend me against my wishes. I had set my preferences to ignore all of this person's posts, but it never occurred to me that I should also specify that no one could friend me without my prior approval! It seems unusual that someone I have never met, spoken to or corresponded with and whose posts I have not read for many months, would want me to be a "friend."