Since “block” represents just the original face, I think you would want the erase “grp” instead.
mod = Sketchup.active_model # Open model
ent = mod.entities # All entities in model
sel = mod.selection # Current selection
grp = ent.add_group
block = grp.entities.add_face([100,100,0],[150,100,0],[150,150,0],[100,150,0])
block.reverse!
block.pushpull 12
block1 = grp.copy
block1.material = "red"
tr = Geom::Transformation.scaling(0.5,2,1)
ent.transform_entities(tr, block1)
tr = Geom::Transformation.new([100,50,30])
ent.transform_entities(tr, block1)
#block.erase!; ## erases the original face only
grp.erase!; ## erases the original group