Stone Wall

Player placed stone wall is the cheap way to shut down spawns in a tunnel or behind a base, and you end up with thousands of the blocks from any real mining trip anyway.

It converts. If Corruption, Crimson or Hallow creeps into range, or somebody sprays the wrong solution, the wall changes type and every room built out of it stops counting as valid housing. Your NPCs pack up without much warning. Green Solution puts it back, but on a base sitting near a spreading biome you are better off with a wall type that cannot be converted at all.