Oh, that.
Well, if you use custom cameras, be sure to set the maintainLOS flag (
http://www.guildofwriters.com/wiki/CameraControls). But I guess you did not use them yet.
The normal camera tries to stay away from colliders, as the avatar does. If it still goes through walls, it can have these reasons:
a) Your walls don't have colliders. Either set the mesh to Triangle Mesh under Logic (F4) => Bounds or, better, surround them with invisible colliders, made of very simple meshes.
b) The area where you walk through is a bit small (e.g. the ceiling is too low or a tunnel is to narrow in width). In this case, it sometimes happens the camera still goes through walls because it's difficult to keep it properly inside a small tunnel, especially when it turns direction with radius too low.
So better try to avoid small tunnels, think D'ni, make your walkable areas large, like D'ni caverns.