Navmesh : suppress area of the navmesh

Hi!

I'm new to unity3d.
I have an agent that I want to make discover the world around him.
For that I just want him to move randomly to area he have never been before. It have a sphere area for its sight radius.
I can take a random point on the navmesh easyly. But how can I supress the part of the navmesh that he have already visit?

The agent is moving on 1 terrain.
Is it possible to put a layer for only an area of the terrain mesh?

 

use http://docs.unity3d.com/ScriptReference/NavMeshObstacle.html or layers.