how to remove terrain textures

Regarding textures,After assigning textures, how do I UNassign them.

I lowered the max height and have that checked in layer painting, to use, so must I repaint all areas to strip materials from high areas of terrain where I do not want them any longer ?

Atm they are still there though I’ve changed it from 1024 to 200 so how do I go about removing them now ?