Non-candidates are the Loophole by Ben S.

In 5x12 the episode entitled "Dead is Dead", when Ben is being judged, "Alex" (MIB) tells him: "I know. Listen to me, you bastard! I know that you're already planning to kill John again, and I want you to know that if you so much as touch him, I will hunt you down and destroy you. You will listen to every word John Locke says, and you will follow his every order. Do you understand?"

This shows that MIB fears being killed by Ben. From what we have seen in Season 6, MIB seems to be immortal (Sayid cannot kill him even after stabbing him in the heart). The reason that Sayid cannot kill him is because he is a candidate.

Meanwhile, the opposite proves true for Ben Linus, who is not a candidate. Ben’s non-candidacy is debatable, the name Linus was found crossed out on the cave wall. I believe however, that the name represents either Alexandra or Roger, who are both dead. Since Ben is not a candidate, he is capable of killing MIB, or Jacob.

The loophole can only be perpetrated by a non-candidate. That means that MIB and Jacob can only be killed by someone who is not a candidate.