What? Didn't you make the map? Each super mirer has 1 tile wide piece of land behind it, burning it cuts a large chunk of land from hostile mire. I thought they were made this way on purpose.
Yes, i made these maps so i can tell you that the super mirer with 1 tile of land behind it you mention are the super struc makers for the red structure line in the maps.
Cutting off these 1 tiles of land will prevent the mire from crossing over. But then the mire will try to find another way so you have to block that off also.
With destroying mirespawners usually i go with the same strategy as stdout's