You must only trust those you can really trust to not grief you. We can't do anything because effectively you gave them permissions to interact in your claim. If you go to any of our /wild warps, you'll see that the claim owner is responsible for its claims and the people they trust.
You must be aware that not every person who joins the server are nice. Those who stay for longer usually end up fitting the community more than those who just join and cause chaos.
Even though we can't do anything about the griefs, we have seen this on the logs:
That person will be punished for disrespect and avoiding chat filter as soon as one of our staff members meet them online.
Thanks for your report.