The libraries of the portable version are stored in the subfolder [font=courier new]data/furniture[/font]. You can remove the files you don't want to keep there.
Note that there are also some Free Art License 3D models in the default library too. If you want to keep only the GNU GPL 3D models it contains, open the [font=courier new]lib/Furniture.jar[/font] file with a zip tool, remove all the [font=courier new].properties[/font] files starting by [font=courier new]ContributedFurnitureCatalog[/font] and save [font=courier new]Furniture.jar[/font].