are you changing the default lambert material'sattributes instead of creating a new one? that could cause your remaining (or new) objects to take on the same material attributes...
in order to remove a material you basically just have to assign another material to it. since there's the default lambert you can change it back to that... (hold right click over the object, choose materials, assign existing material, then choose lambert1... or just create a new material.
Accept no substitutions.