Hallowed Grass Wall

The wall that grows naturally in the Hallow pushes the biome outward into clean walls next to it. The one you buy and place yourself does not. They look identical, which catches people out: converting an existing wall with a spray gives you the spreading kind, while buying the item from the Dryad gives you the inert one. For anything you are building on purpose, buy it.

The Clentaminator sprays a wide column and it does not distinguish between walls, blocks and plants, so a light touch is hard to manage indoors or near a build you care about. Holy Water is slower and less convenient, but it lands where you throw it, which is what you want when the target is a few walls in the middle of something else.