Harassment and Invincibility

Hello, there is a player on my server who harasses my alliance and sends death threats by targeting small players as a priority, all alliance of my server have orders not to recruit him, despite this he continues. And what’s strange is that I only have 3,000 power differences and my attacks don’t do anything to him even in rallying, there are only 1 out of 3 attacks that attack its walls and its men but the second after all is regenerated there are only players with twice its power that manages to do damage to it and unfortunately less than normally. What should I do?