Well it is at it's core a Co-Op game I would say put a password and admin a server. Open servers this is an issue. However you can still kick people that do this I believe.

At this stage of development there are almost no server settings at all that you would see in other games of this style. I don't know if the Devs plan on adding them. Of course no player damage just opens you up to a new issue. Lets build a big wall around the elder... or Bonemass... etc. I realize there is always more than one spawn for this... but Grievers will generally find away to grief and the harder you make the more they will try.