The only Njerp warcamp I've ever seen is the one you start in with the escaping slave scenario. They are supposed to exist (Sami hasn't removed them), but I haven't seen anyone on the forum claim to have seen one since I started with UrW a number of years ago.
Culture control is based on villages of that culture exerting influence on the surroundings, and that village is typically close to the center of the region "block", although there may be several villages within a block, in particular clusters connected with roads.
There should be a village in the region that formerly belonged to your culture (or just inside the border of the current region, if the part lost isn't a whole "block", and that village ought to be abandoned if inside the region, or either abandoned or populated by "vagabonds" if outside.
There was a bug that caused depopulation of a village to sometimes remove the culture as well as a neighboring unvisited village, which then was replaced by vagabond settlement (one or a few tents and a small family), but I think that one was fixed (in my case it was my character's doing: trying to purge the world of Njerps, with the expected result: eventual death).
Anyway, I haven't seen it happen spontaneously, or heard any other case of it happening, but Njerp or beast attacks, and possibly robbers should be capable of doing it. I think you can be reasonably sure the biggest threat to your culture is your character, and if your character isn't bent in its destruction it should remain (although UrW currently doesn't have any repopulation mechanism, so every death is a permanent loss, which means gradual depopulation of villages over the very long term).