If all these explorers are able to share this rumor so fast and block the griefer, why don't they just form a private group?
Short story:
Once upon a time, there was a sandbox where all the kids could play in together. Everything was fine and dandy and all the kids were happily playing, building sandcastles, digging holes and whatnot.
One day, two kids came along to the sandbox where all the other kids were playing. These two kids started pushing the other kids around, peeing in the sand, depositing poo in the sand, and ended up punching and kicking the other kids.
This made the other kids very unhappy, because no longer could they play together in the open sandbox without these two kids arriving and ruining everything for the majority of the kids.
The larger group of kids told the owner of the sandbox about this behaviour. The owner thought the two kids were ruining the fun of the majority of the kids who used to enjoy playing in the sandbox. So they set up a way of preventing the majority of kids who didn't want to play with the two unruly kids, from meeting with them.
Eventually word of this method got around, and all the kids who previously were playing happily together, started using this method. No longer would they encounter the two unruly kids in the open sandbox.
What's more, because it was an open sandbox, even more kids could come and play, and they had the choice of being able to meet the unruly kids, or not, or change their minds on that whenever they liked.
The two unruly kids, however, were unhappy. They didn't understand why the other kids didn't want to meet and play with them. For these two kids, kicking down sandcastles, peeing and pooing in the sand, and kicking and punching the other kids was fun, for them - after all, that's what an open sandbox is for, right? So the two unruly kids eventually had no other kids to 'play' with, and there they were, isolated and alone in an otherwise empty sandbox.
They never did understand why it ended up so empty, for them.
The end.