From what i have been checking in a no zombie kill playthrough, as well as looking up info in the console, there is an interval after defeating a WH when a next will not spawn. This allows time for killing them and not having 2 at a time. Additionally, every WH zombie dies after a period of time after spawning (seen it in both A16 and A17). Apart from that, even if you sneak around, WH will follow players general direction (unless they changed it, after seeing them n the horizon, i hid countless times in a base on stilts and they came from various distance to me, slowly creeping all together). So unless you run/ride far enough to despawn the whole group or wait for them to die, you need to kill them manually or with traps.