PROBLEMS WITH DESERT FOSSIL WALLS

[Den]

So right after hardmode, the desert in my world got converted into hallow. The problem is that I can't stop it from spreading through my underground desert, because I can't mine the DESERT FOSSIL WALLS. Do you guys have any idea how to stop the hallow from spreading through these new walls? Would jungle blocks work? thanks!

[Crucible]

Get a bunch of bombs and start bombing a hellevator from the surface. Natural walls can only be removed if you start at the edges.

Tip: If some bombs miss the walls and fall/detonate past the edges, place a block near the edge of the wall and plop a Sticky Bomb onto it.