Crimstone Wall

The wall the world generates counts as unsafe, which means enemies spawn happily in front of it. A wall of the same name that you place yourself is inert and blocks spawns like any other player-built background. So a room carved out of infected ground is only safe once you have torn out the original wall and put your own copies back in the holes.

Naturally generated crimstone walls push the infection outward on their own. This is the part people miss when a containment trench fails: every block was mined out, the tunnel is clean, and the background is still quietly converting the stone behind it. Clear walls as well as blocks or the trench is decorative.

The Clentaminator sprays wide and will convert blocks and neighboring walls along with your target, which is a problem when you only want a themed corner in a build. Blood Water thrown by hand is the precise tool for that job, slower but controllable.