Charanjeet,

That depends on what you want EXACTLY.

1 - Fix walls so they won't be accidentally deleted: use Plan --> Lock base plan
2 - Fix walls so users can change the entire plan, but not the walls: not possible
3 - Allow users to view the model without changing anything: use the viewer

There is, however, a workaround.
- Install the plug-in CopyAsNewFurniture (see tutorial )
- Make your design with walls, floors and roofs, and save it.
- Insert at least ONE piece of furniture (doors and windows also count as furniture)
- Use Tools --> Copy as new furniture to copy your design.
- Open a new instance of SH3D and paste your new furniture into it.
- Use this fixed model to make the rest of your design.

Good luck

Hans