Just had an idea, and sorry if it has already been put forward, but:
In J.R.R Tolkien's 'The Lord of the Rings', the One Ring can only be destroyed where it was created. So:
We have seen killing MIB isn't exactly the simplest, easiest thing to do, but: what if Jack, or whoever becoems the new Protector (Jack!) will have to take MIB and throw him down into the Light, similar to how Jacob did?