I'm not sure if there's a way to "re fog" unfogged areas. I suspect not though.
There is in the atmosphere settings a thing called distance fog. You could make it extremely thick and dark and make the near distance really close to the player, meaning you can't see far.
One of the built in atmospheres has this by default:
ARX_HorrorSleep_Cave_PitchBlack
But it's REALLY dark. It should be possible to do it in the light but you might still be able to see too well for your purposes. Also if someone enables character outlines that probably defeats the purpose as well.
