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. Using save file modification, it is possible to have "delete particles" on your screen, but they disappear as soon as the game is unpaused.
When used with the Replace tool, you can safely remove particles of Rigid without breaking your creation.