I think it would be a decent idea if HNMS required a huge number of people to kill ... 15+ and then once Your character has killed said HNM, you get locked out from being able to claim it again for a few days or whatever time period is deemed fit. This way the same people/group can't keep camping the HNM and killing it over and over again. The same could apply for reg NMS as well, but a lesser duration.