Autodesk's Jerry Bartels demonstrates the use of the Map 3D MAPTRIM command to clean up an Exported Civil 3D model.
Jerry also covers how to employ Layer properties to include and exclude AutoCAD object content from the MAPTRIM.
and the Civil Immersion blog.
MAPTRIM and MAPCLEAN are two of everyone's favorite Map 3D tools that a skilled Civil 3D user has to know.
Needless to say, this is a publication process. How well you set up the published content before you perform the MAPTRIM is important to produce good results.