Opensim has traditionally been considered by many resident's as being griefer free, or close to it. This is not the case recently, unfortunately one or more bad actors are spreading pain around the Hypergrid. Why they do it is not worth debate, what is worth talking about is how easy it can be to thwart, discourage and prevent it from happening to you. I am assuming that you have estate manager privileges and the land is set to a particular group with by invitation only group membership.
Once all of these parameters are verified, simply go into your about land settings and select options, in the everyone section turn off the build, object entry and run scripts check box's.
This is your best and easiest way to implement a solution that I know of.
JayR Cela :_)