I have committed Njerpocide. All villages and camps wiped out as far as I can tell (new camps were popping up until the last village was razed so it is possible that I've missed one or more). I continued play for a couple of years and based myself in the former Njerp lands, and have not seen any wanderers. So yeah, I think they stop spawning. I kinda miss the little red fellas now!
I sometimes found wandering Njerps within a few tiles of a camp, and if I killed them the camp had fewer warriors in it than the usual when I attacked it. It seems that some wandering Njerps are wandering around in twos or threes close to camps, and others are wandering the wilderness and spawn at random - now weighted towards spawning near Njerp territory rather than all over the world as was previously the case.