What you want to do is not what Facebook thinks you should be doing... Facebook is after all a social network so it goes beyond 1:1 and has grown to be very sophisticated at suggesting people around. There's no perfect way to stop it since that is the goal.
What you can do, as Felgall mentioned, is make your account basically unfindable. If you have no friends, no one can see you unless they search by name.
If you have a few friends there will be some suggestions made and you will get requests. You can't block them up front but when you decline nothing is sent to the user, they merely don't become your friend and may try again down the road.
Lock down your privacy settings so no one but friends see anything and you'll see the asks get reduced.