There is this Supreme guy named Clone66, He likes to be added to plots and grief them, especially to his friends plot. If you look around the 'Reports: Banned' section, you will see a report made by @redroyalle of Clone66 griefing. But apparently, he didn't got banned.
It's a matter of trust, I don't know if there is a punishment for griefing given that it was the players choice to accept Clone into the plot.