I agree with the thought of having S ranks immune for a short time after spawning, say 5-10 minutes. This would allow the spawners sufficient time to locate, and relay the hunt to others, and for others to actually arrive on time to get the kills. This doesn't prevent anyone from getting the kills and doesn't punish wanderers. In fact, it would allow wanderers more time to arrive since often there have been queues to go to other worlds that are long enough to cause people to miss out. In my mind, this so far has been the simplest and probably easiest to incorporate suggestion.