Upon using Explode command I was able to delete the not required parts. Thanks!
At this moment, I don´t want to remodel the pylon in nurbs. I just want it for some landscape in GrassHopper.
However, the bad (invalid) objects produced problems in GH. The model was loaded fine (even it could be rendered), but the bad objects were not properly processed by GH, e.g. only good objects were moved.
To solve this issue I followed some tips for bad meshes from Using Grasshopper to Fix/Simplify Complex Invalid Mesh and https://wiki.mcneel.com/rhino/badobjects
If a polygon mesh object rings up as bad, use Check command to see a more detailed report of what is wrong with it. This command tells you a good deal about what can be fixed, and you may find the commands you need among the offerings in the Mesh menu, under mesh Edit tools and mesh repair tools. Some useful commands are: ExtractDuplicateMeshFaces, ExtractNonManifoldMeshEdges, CullDegenerateMeshFaces, and UnifyMeshNormals.
In particular, _RebuildMeshNormals, _UnifyMeshNormals, Explode, ExtractDuplicateMeshFaces, and CullDegenerateMeshFaces commands were applied, but I think the later was the one that did most of the job for this pylon.
Thanks all guys, very kind, quick, and useful information!