Do any harvestable natural walls spawned at world gen count for stopping monster generation?
From the wiki:
Natural walls will not stop enemies from spawning, and cannot be utilised for Houses.
Also from the wiki, on destroying and placing natural walls:
Certain walls, [] exist in both natural and housing forms. With these walls, the player must break the natural walls and replace them in order to allow them to count as housing walls.
This means that the type of wall has no impact on monster spawning, all that matters is whether or not it has been placed by a player. As housing walls do block monsters, and natural walls do not.