Delete is a pseudo-material used to remove another material. It's not really a material, it doesn't burn, melt, evaporate, float, or sink, and it does not produce visible particles. Any material containing "Z" or Delete will instantly disappear when drawn. If you force it to Pour with shift-Z, the material that pours is Null.
When used with the Replace tool, you can safely remove particles of Rigid without breaking your creation.