I'm sure there is more than one way to do this, but the first method that comes to mind is to create a point group which excludes the elevations you are talking about, and then define the surface using that point group. If you already have the surface created and you don't want to make a new one, you can go into the surface properties->definition tab->build->turn on (change value to 'yes') the 'exclude elevations less than' option and enter the value you want to exclude. When the surface is rebuilt, points with those elevations will be removed from the surface. Hope that helps.
LDD? Sorry, I haven’t used that in about 15 years and I have practically zero recollection of how it works. LDD and Civil3D are related though, so perhaps the sequence and logic of how it’s done in Civil3D might give a hint as to how it’s done in LDD?
