Any idea how I could restrict access to certain parts of an open space to SOME NPC and forbid access to others ?
example : right part of the room is for males, left for females, and they do no cross the line.

I believe a navmesh only has the parameter to restrict large creatures from certain areas. I don't think that can be modified to include other NPCs.
X Markers have certain flags you can set but again there isn't a setting to restrict a particular type of NPC.

Depending on how the room/area is set up and what else you have the NPCs doing, one suggestion is creating two different sandbox markers on opposite sides of the room and restricting the radius so that they dont cross into the opposite area. This obviously isnt perfect because it deals with a circular area but thought I'd throw it out there.

- Mal

Ownership might be something to experiment with. Set up a 'left side' faction and a 'right side' faction and then mark the ownership of all of the furniture and idle markers appropriately.

Thanks for the useful suggestions.
I was looking in a wrong direction with trigger zones and collision boxes.
I think A mix of both ideas could lead somewhere. Scripted markers and idlemarkers belonging to a faction with a patrol path and a small wandering radius should keep the public out of the crime scene

Another possibility did occur to me, although it's a bit more restrictive. You could just split the navmesh down the middle (to use your 'left side/right side' example), leaving only a join between the two halves at some point away from where your NPCs are gathered. They would still be able to get from one half to the other if needed, but not directly. A bit like navmeshing two rooms side by side but with no actual wall between them. As long as they are not allowed to wander near this 'bridge' between the two sides then they shouldn't cross over. The problem with this idea would be that if you did want your NPCs to move between the sides at some stage then they would take an odd route to do it.

The problem with the navmesh solution is NONE of the NPC will cross the line on their own wandering package. What I want is some NPC (guards) wandering freely in all the area but he faction townies stick on their own side. In Oblivion it was possible using markers but I didn't find how to do the same in Fallout.

I think the owned activators is the best solution. The left activator can bear a script to detect the approaching NPC and add a scrip package to direct it to the right activator if he doesn't belong to the left faction. Then the right activator will replace the package . With owned furniure and objects on each side they will not be much interestd in the other side and wih the acivator they will be bounced if they wander near the line. It is not perfect but it would be enough to give the impression the NPC are truly separated. I'll have to test it...