Moderators, be aware of ceirtan really bad griefing groups.
I recently used 3 hours building a building, then a group of griefers (3 or 4 players) came and started tearing it apart as I was still building. I yelled at them, and they started spamming that I was griefing.
In other words, be aware that groups of such @holes exist, and don't blindly trust anyone before start considering banning people. If the server is properly logged, it should be no problem finding out who was greafing or not.