Just to clarify, it was ash, not salt. We were never told/shown who broke the ash circle but it’s presumed the MiB either secretly manipulated Ben into doing it or Smokey found a way to do it himself, just like he was able to circumvent the ash circle around Bram by making a chunk of the ceiling fall down on him. If Smokey, the exact method will never be known but given he was able to use that loophole to kill Bram, there’s no reason he couldn’t have thought something up put a break in the ash.