Unfortunately, you can't trust many people who are online nowadays. The person who did this is someone who checks on the siren board, since there isn't really a way of finding it otherwise.
You'll unfortunately have to stick with keeping it private, and only inviting people you can trust to work on it.
Why do people grief literal maps anyways? It's the dumbest way ever of getting attention.
EDIT: I just got rid of some of the griefing that was done.